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Xnke
12-03-2017, 12:54 AM
Still saving money for the HMG flat and weldment kit...hopefully by the time I can save it up they still have some. If not, well...I have made dies before.

Anyway, what I want out of my L kit is a "DMR" style rifle. Think Cetme L meets PSG-1. We'll see if I actually make it happen or if I just build a standard L, but here is the "desirable" list.

-18" free-floating barrel
-Integral bipod
-Birdcage style flash hider/muzzle device
-Removable optic
-Good iron sights
-400yd effective shot range
-6.8SPC chambering

Here's the "obstacle list".

-It's roller delayed.
-Front sight base is pinned to barrel
-Handguard pinned to FSB
-Sights sit very tall on receiver
-.223 is boring, but cheap and barrels are readily available.
-6.8SPC is neat, but fluted chamber barrels are only made from the purest unobtanium.

Now, I don't get to go shoot as often as I'd like these days-I maintain my range membership but maybe 10 days a year do I get to actually go shoot. My range has a 385yd limit, that's the longest target frame distance-and it's uphill. I *love* being able to go out and tag the 4" bell at 385 with most of my rifles.

So, since I don't have the receiver bits yet, but I already stripped down the old receiver bits following the HMG instructions, I thought I'd look at the free-floating barrel issue. This is the steel handguard liner, cocking tube, and front sight base all layed out together in roughly the positions they will be in:

https://i.imgur.com/TWzmtrA.jpg

and the disassembled view: (We prefer to not use the term "exploded view" when dealing with firearms, gas appliances, or auto engines...)

https://i.imgur.com/poIp7mx.jpg

The steel handguard liner didn't want to lay still for the photo. Drill bit helped.

I am thinking maybe weld or pin the cocking tube to the front sight base, and bore the barrel ring slightly larger or turn the barrel slightly smaller (probably turn the barrel!) to provide for the free-float feature. The cocking tube is a substantial bit of steel, but I think it will need some help from the handguard-maybe a pin through at the bottom of the front trunnion? (I'd have to weld an extension block to the front trunnion to hold the pin, but that's nothing too difficult.

Another thing is the 922r parts count-would anyone else be interested in some forearms, pistol grips, and maybe buttstocks? I am planning to have a go at molding some, to get my US parts count up. I prefer to stay under the "10" imported parts by at least two. Also, the PSG-1 is wildly different in those areas, so why shouldn't mine be? Even if all I do is mold them in some color other than dirty green...Personally I like the duracoat #80 "HK Blue/Grey" color...

As for the barrel, there's no way like the right way to cut chamber flutes. My test Method #1 was a plastic chamber mold plug, with grooves carved in to lay a brass wire, an o-ring seal on the freebore area to protect the rifling, and a bit of plastic tubing to provide a "flow-through" style flush and electrochemical machining should be the way to go. The only issue is, my test piece didn't cut the flute deep enough-I haven't provided a way to "advance" the brass wire "cutter" to keep cutting. Finish was excellent, but the depth of the cut was only 0.008". I am afraid that unless I can keep the gap to a controlled size the flute width will be hard to control. Method #2 of cutting the flutes was to paint the chamber, and then scrape out the flute area to bare steel again. Once the 12 flutes were scraped down to bare steel, a cotton plug was used as the electrode, and a steady stream of electrolyte was used to keep the plug wet and cooled. I was able to etch the flutes to 0.01" deep before undercutting started to become very evident. Maybe rough in with the etching method, then finish up with files? Not sure.

Anyone else play with the electrochemical method to cut the flutes?

pryotex
12-05-2017, 07:11 AM
Holescreek made a cutter for the chamber flutes and cut them individually. He cast a mold when he finished and it looked pretty darn good.

He's a member here, Hit him up on that. Why re-invent the wheel.

Mike928
12-05-2017, 08:45 PM
Still saving money for the HMG flat and weldment kit...hopefully by the time I can save it up they still have some. If not, well...I have made dies before.

Anyway, what I want out of my L kit is a "DMR" style rifle. Think Cetme L meets PSG-1. We'll see if I actually make it happen or if I just build a standard L, but here is the "desirable" list.

-18" free-floating barrel
-Integral bipod
-Birdcage style flash hider/muzzle device
-Removable optic
-Good iron sights
-400yd effective shot range
-6.8SPC chambering

Here's the "obstacle list".

-It's roller delayed.
-Front sight base is pinned to barrel
-Handguard pinned to FSB
-Sights sit very tall on receiver
-.223 is boring, but cheap and barrels are readily available.
-6.8SPC is neat, but fluted chamber barrels are only made from the purest unobtanium.

Now, I don't get to go shoot as often as I'd like these days-I maintain my range membership but maybe 10 days a year do I get to actually go shoot. My range has a 385yd limit, that's the longest target frame distance-and it's uphill. I *love* being able to go out and tag the 4" bell at 385 with most of my rifles.

So, since I don't have the receiver bits yet, but I already stripped down the old receiver bits following the HMG instructions, I thought I'd look at the free-floating barrel issue. This is the steel handguard liner, cocking tube, and front sight base all layed out together in roughly the positions they will be in:

https://i.imgur.com/TWzmtrA.jpg

and the disassembled view: (We prefer to not use the term "exploded view" when dealing with firearms, gas appliances, or auto engines...)

https://i.imgur.com/poIp7mx.jpg

The steel handguard liner didn't want to lay still for the photo. Drill bit helped.

I am thinking maybe weld or pin the cocking tube to the front sight base, and bore the barrel ring slightly larger or turn the barrel slightly smaller (probably turn the barrel!) to provide for the free-float feature. The cocking tube is a substantial bit of steel, but I think it will need some help from the handguard-maybe a pin through at the bottom of the front trunnion? (I'd have to weld an extension block to the front trunnion to hold the pin, but that's nothing too difficult.

Another thing is the 922r parts count-would anyone else be interested in some forearms, pistol grips, and maybe buttstocks? I am planning to have a go at molding some, to get my US parts count up. I prefer to stay under the "10" imported parts by at least two. Also, the PSG-1 is wildly different in those areas, so why shouldn't mine be? Even if all I do is mold them in some color other than dirty green...Personally I like the duracoat #80 "HK Blue/Grey" color...

As for the barrel, there's no way like the right way to cut chamber flutes. My test Method #1 was a plastic chamber mold plug, with grooves carved in to lay a brass wire, an o-ring seal on the freebore area to protect the rifling, and a bit of plastic tubing to provide a "flow-through" style flush and electrochemical machining should be the way to go. The only issue is, my test piece didn't cut the flute deep enough-I haven't provided a way to "advance" the brass wire "cutter" to keep cutting. Finish was excellent, but the depth of the cut was only 0.008". I am afraid that unless I can keep the gap to a controlled size the flute width will be hard to control. Method #2 of cutting the flutes was to paint the chamber, and then scrape out the flute area to bare steel again. Once the 12 flutes were scraped down to bare steel, a cotton plug was used as the electrode, and a steady stream of electrolyte was used to keep the plug wet and cooled. I was able to etch the flutes to 0.01" deep before undercutting started to become very evident. Maybe rough in with the etching method, then finish up with files? Not sure.

Anyone else play with the electrochemical method to cut the flutes?

Do you mind sharing some pictures of your "failed" attempt at chamber fluting?

holescreek
12-05-2017, 11:06 PM
I've always wanted to try ECD chamber fluting. I've been involved with discussions of it on several boards over the years and it usually ends up that not enough people have hands-on experience with it so the details get muddied quickly. HK has been ECDing flutes forever but they have specialized equipment for the task. One thing realized early on is the issue you've run into trying to erode all at once. There just isn't a good way to deepen the flutes without eroding chamber material around the flute. I think the solution is to make an electrode that only does one flute at a time so it can be fed to depth, the rotate the barrel 1/12th and start over again repeating until the chamber is complete. With ECD the electrode never touches the work so it should last for a very long time.

The last issue (for me) is coming up with a way to measure flow through the flutes before removing the barrel from the fixture to know if the fluting job is good enough to work. Regardless of the method used it sucks trying to set a barrel back up to make flutes deeper. At the same time flutes that are too deep usually scrap the barrel.

Blackwing
12-06-2017, 12:09 AM
Do you mind sharing some pictures of your "failed" attempt at chamber fluting?

+1

Xnke
12-08-2017, 02:27 AM
It's been two years since I did the chamber fluting-I have my notes but it may be a task to find the actual chunks of barrel that I played with. I'll likely just have to give it another go and see what happens-the biggest issue with ECM'ing the flutes in isn't making the cutter, or building the fixture to do the indexing and cutter advance, or even the power supply-it's the electrolyte flow rate that seems the big hang-up. Every reference I can find (including references on machining the entire chamber!) says to keep the cutter within 0.003 to 0.005 inches of the surface to be cut, and to provide 200PSI electrolyte flow to ensure complete flushing and cooling of the electrode. Removal rates at 10A and the surface area we're interested in are in the ballpark of 0.020" in 10 to 15 seconds using sodium chloride electrolyte. My thoughts are a plastic rod, with a bit of 0.005" brass shim stock embedded in it, for the cutter, and blast electrolyte down from the muzzle end of the barrel out the breech as high a flow rate as possible, and hope for the best.

From what I've read, the flutes should be 0.020" wide, and 0.020" deep, and extend from just beyond the cartridge neck down 80% of the length of the casing.

The reason I don't go for the holescreek method is that I do not own a CNC milling machine to do the work for me. I am building a CNC plasma table to try and get some money back out of my toys, but no CNC mill yet.

Xnke
12-09-2017, 12:48 AM
So to all those out there that feel the Cetme L is inferior...it's SUPER overbuilt compared to the C version. Just went through all my kits parts and every detail of the C is just battered and bent, while my L kit is very straight.

In my C kit, the cocking tube is very thin and mine is bent and cracked, wheras the L tube is MUCH thicker and very straight, with no damage I've been able to find.

In my C kit, the front handguard is usable-but cracked, and the way it's cracked is kind of unavoidable due to the push-on steel furniture on the ends. Now, I'm a wood-and-blued-steel kinda guy, but my C kit may wind up a C-turned-HK91 build...this wood stock is ROUGH. We'll see how it cleans up.

In the L kit, the plastics are in good shape-I can carefully scrape and polish these back up to a nice enough finish to make molds off of them. That'll be 3 more 922R parts out there!

11C ABN
12-09-2017, 03:57 AM
Might I make couple of suggestions?

The cocking tube and sight fixture which secures to the barrel should be modified to meet your prerequisites. The barrel your intending too use will be a bull barrel? The CETME L Sight blade sits very high as to mimic an M16A2weapons platform. This being said the PSG1-A2 weapons platform is something you should look at.

The squared cocking tube would allow you to weld a 3/8inch inch piece of steel flat stock to the top of it insure it extends the length to the end of the cocking tube. The front sight base will need to be milled/cut down level with the cocking tube. (Keep the Portion of the sight base that’s been milled/cut off and place to the side)
The extension of the flat stock from the cocking tube should lay flat on top of what you’ve milled/cut from the sight base. Drill/Tap the flat stock to attach to the sight base portion.

Contact Member Turbo this and purchase an 18 inch steel 1913 pictinny rail.

Cut the rail to length and drill/tap to attach to the 3/8 inch welded steel flat base that’s been welded to the cocking tube and screwed to the sight base.Here’s were you can think’ use the existing sight base and attach to fabricated attachment. Sight can flip up and/or be stationary. You make the call. FYI you can mill the 1913 pictinny rail so the flip up sight sits in a milled out slot of 1913 steel rail.

The barrel is know free-floated from the sight attachment.

Measure the clearance from the bottom of the cocking tube, insure your bull barrel fits

The hand-guard is going to need modification to fit the new diameter of the bull barrel, an attachment method will need to be fabricated to attach to the cocking/tube sight base

Xnke
12-10-2017, 04:41 AM
The barrel won't be a true "bull" barrel-it will be cut to a stepped non-tapered profile. Think Old Mauser military barrels, but the heaviest profile that will fit through the trunnion, and stepped down once to the max diameter that could fit through the handguard without touching, and then maintain that diameter up to the threading point for the muzzle device. This way, I can turn a nice balance between total weight, barrel weight, and balance.

I figure I could do something similar to what you suggest. I was thinking weld the steel front sight base to the end of the cocking tube, and install a steel rail along the top of the receiver and cocking tube. It'll have to be set up once I have the new plastics made up and able to be bolted to the receiver so I can set up the sight lines and decide what I like and what it really needs.

Xnke
07-11-2018, 03:25 PM
Been a bit since I looked at this. Got a new job, then another new job, and now starting a third new job this year, all a little better than the last, so not much has happened with the 6.8SPC Cetme. I have finally obtained the HMG flat, weldments, and doing and screw kit, so now I am only short a barrel, the HMG folding jig, and the HMG disconnector of having ordered all my materials. I am still planning to mold a new set of plastics for this rifle, but since the shop is 90+ degrees this time of year, it will have to wait a bit.

scottz63
07-11-2018, 04:48 PM
Looking forward to seeing more progress on this. :)

Xnke
07-11-2018, 10:50 PM
Pulled out the plastics again, and am studying up on how to mold them properly. The steel handguard liner still needs to fit properly and I still need to have the brass inserts in the right spot, so I am not so sure how to make that happen yet.

rebel49
07-12-2018, 01:06 AM
You may want to contact Rim Country Manufacturing in Payson, AZ. for a barrel. They make barrels for HK products and I believe they flute the barrels. If you knew the contours of the orig. BBL they could very possibly make one for you. They did a stellar job on 2 Steyr M357 barrels for me. Just a thought.

scottz63
07-12-2018, 04:11 AM
HMG makes them.

https://www.hmgunworks.com/product/hmg-cetme-l-barrel/

Xnke
07-12-2018, 01:51 PM
Neither of those companies make a 6.8 SPC barrel. I am building this rifle in 6.8 SPC, and as such will need to make the barrel and locking piece in house.

For the locking piece I am still working out what the new angles should be, I think Holescreek had the formula to guesstimate the angle needed posted here somewhere.

6.8 SPC 10rd magazines showed up today. I am torn between using a green mountain "Gunsmith" barrel for 60$, or splashing out 400$ on a Lilja 3R barrel. I can get a 25" barrel blank from either company, in a 1:11 twist. Both will need to be profiled and chambered here, so I can't really see spending the extra money for any other reason than "Oooooh Shiny" reasons.

biffj
07-13-2018, 11:53 AM
Chamber fluting is really important for operation. Good flutes will allow your rifle to not only operate properly but will not damage the brass. Bad flutes may allow operation with some types of ammo like steel cases or especially hard brass but you'll find softer brass sticking or flowing into the flutes ruining the case and slowing the action down. Having built a goodly number of roller delay blowback rifles with a variety of barrels I can tell you that it makes a huge difference. The green mountain barrels and some that may have come from PTG had a lot of problems. My first HK 33 would only work with steel cased ammo and had a GM barrel. The last one has an RCM barrel and works with any ammo I've tried. The same is true of the G3 and Cetme L. The main point of failure seems to be the shoulder area. If the neck and body flutes are ok but the shoulder is shallow or narrow or anything that restricts flow you'll have trouble. The idea is to float the case with equal pressure inside and out. If you have insufficient flow through the flutes the case material will flow into the flutes blocking them further and causing failures. The shoulder is the hardest area to cut and frequently is not deep enough. That chokes the flow and makes the body flutes useless. Fluting is really important.

You can or will be able to buy new American made plastic for the L from MarColMar when they have production up. I've seen and handled test pieces of the stock, handguard, grip and cocking handle and they look really good. They will have black, flat dark earth and green furniture at some point. There is a thread here concerning their project-
http://www.militaryfirearm.com/Forum/showthread.php?34981-MarColMar-New-CETME-L-Rifles
There are a lot of issues with molding that aren't obvious til you try it. A lot of things don't become really usable until you have enough production to pay for it. Fitting the brass inserts into the parts is one thing like that. Making up the fixtures to hold the inserts for molding or to install them is another of those things.

I shoot a 6.5 Grendel in an AR and looked at the 6.8 for a bit. Both interesting cartridges that make an otherwise uninteresting rifle worth fooling with. (not a big AR fan here) On the other hand the 6.8 is going to require that you open up the bolt face on the L to handle it. There isn't a lot of room there in the bolt and also the extractor will require mods to work right. I don't know if the L bolt is case or through hardened and that can affect the mod quite a bit. With case hardening it might weaken the bolt face and cause failures. My guess is that it is through hardened of some special alloy so maybe that won't be a problem.

The front sight base is unimportant as far as operation goes. You could easily make up something like the PSG uses and attach it to the cocking tube. You could just leave it off and attach a rail to the receiver instead of the rear sight base. It would probably work better anyway. The only real purpose served by the sight base is to attach the handguards front end. Chances are you won't be shooting rifle grenades and if you're looking at a heavier than normal barrel which is a really good idea you will end up cutting through the bore of the original sight base anyway. Attach it to the cocking tube as a front hanger for the handguard and be done with it.

You could talk to Holescreek about gas operation and screw the fluting altogether. I can't see any reason why you couldn't go that way. Its one of the projects I've had planned for a G3 and maybe an L. The Germans at Rheinmetall did it with the original G3 design in 7.62X39 in the 60s. Lack of interest is the only thing that kept it out of production.

Hope something there helps.

Frank

Xnke
07-14-2018, 12:23 PM
The brass inserts are easy-you use a silicone skin mold over a round plastic tube, I don't need to tool up for full injection molding. Cast resin will work fine. The rigid plastic tube holds the screws in place, then the brass inserts are threaded on. The mold is closed, poured, and the brass is cast in place. Once cured, the screws are removed, the plastic tube is slid out of the silicone skin mold, and the part is unmolded. Or, alternatively, the brass inserts are soldered to a wire or strip, placed on nubs in the silicone mold, and cast in place as a unitized structure. This also adds wire reinforcing to the casting and positively prevents spinning the brass insert in the end product.

I used to do low-volume production castings in aluminum, for hot-rodding. Intakes, bellhousings, and valve covers, all done in sand molds. Never done a LOT of silicone molding but there is a local prop shop that I have been getting information from. We'll see how it goes.

As far as the barrel, I am piddling around with electrochemical machining for this one-I was just going to do the flutes via ECD but it may be easier to do the whole chamber in one shot. The current working idea is a fluteless brass chamber reamer, with a plastic pilot, and a linear motion stage for the insertion/removal. I have worked out the flow rate issue and have a Procon carbonator pump pushing the sodium nitrate electrolyte from the muzzle end of the barrel up through the barrel, washing the chamber out as the tool is advanced. This produces a very nice chamber, but no flutes yet. Working voltage is around 1.1 volts, and working current 56A to cut a chamber in under 2 minutes. Chamber sizing is a bit touchy-if the voltage or current varies even just a few percent, the feed rate changes a lot in order to keep the chamber in the proper tolerance. And if the tool touches the barrel...well...time for a new tool and a new barrel.

So far, I have not gotten any of my all-at-once fluted chamber tools to work. They cut flutes...but HUGE flutes. Like dodecahedral chamber shaped flutes. If I come in with a second pass tool, I can get the flutes to work out better, but with all the job hopping I haven't gotten a single chamber fluted properly yet. It's a time vs desire problem at the moment...the workshop is 100+ degrees inside until September rolls around and it cools off.

If I can make the flutes work properly, I am considering the difficulties in ECM'ing rifled barrels...custom twists and land/groove configs? No problem when the tools never wear at all...the twist rate is generated on the fly by the rotary machine, so you just have to make a brass button with the land/groove config, then roll the barrel as the tool is drawn through, just like a sine bar machine...but CNC. So gain twist, delayed twist, 1 in 100 all the way to 1 in 1 twist...should all be doable with an atomically fine finish. The cost of electricity to run the machine though...not so economical.

biffj
07-14-2018, 11:58 PM
We want to see what you come up with when you get there. I've seen a couple different ways to do the flutes and they can all work. Keep up the work and get er done.

Frank

Xnke
07-15-2018, 06:10 PM
So I went for a swim (meaning I went outside to the shop) and did some work on the boltface today-it's opened up to 0.423" and I have not modified the extractor yet. The extractor spring...geez what a pain to deal with!

Before modification:
https://i.imgur.com/03HnEWI.jpg

https://i.imgur.com/LgfbxMf.jpg

After opening the bolt face, but prior to polishing:

https://i.imgur.com/mvQuEoG.jpg

https://i.imgur.com/kdpIZEF.jpg

I'm going to polish the bolt face up a bit, and lightly radius the edges of the recess, to make ejection easier on brass. The extractor is very tight on the base, but it was just as tight on the .223 cases before modification, so I am not sure if I should work on that any or not. I guess we'll find out after the rifle is assembled and I can let the bolt fly forward on spring pressure-I am familiar with other semi-auto rifles desire to fling the bolt forward in order to hook the extractor.

Xnke
07-18-2018, 10:38 PM
I seem to have misplaced my locking piece for this rifle. That's unfortunate, since I need to make a drawing of it to make a new locking piece. I'll order a spare tomorrow.

Holescreek, I have to make a few locking pieces. The machine shop I work at absolutely will not allow any employee to make or modify any parts for themselves, nor are we allowed to have the shop make them for us. Would you be interested in the grinding work on a Cetme L locking piece? I will do the lathe turning and milling here, soon as I work out how to do the tab. (pretty sure I can file it into shape easily enough.) It is much smaller than the Cetme C locking piece, and I am not sure yet what kind of angles I will need...because I need to measure the existing LP first.

biffj
07-19-2018, 01:13 AM
The existing locking pieces are 50-53 deg. I can get a few on loan to Holescreek if that will work for measuring. Not too far from the dayton area here.....

Frank

holescreek
07-19-2018, 10:29 AM
Sounds like you have it figured out but but not having a Cetme L puts making parts for one at the bottom of my interest list. It also makes grinding the shoulder angles to the correct height impossible without a bolt head and trunnion measurements.

Xnke
07-20-2018, 10:56 PM
Understandable. Before I go and assemble this, I need to get those bolt head and trunnion measurements-essentially I need to duplicate the process you outline in your locking piece thread. How did you measure the bolt head, trunnion, and locking piece to determine the datum lines for establishing the locking piece widths? the L LP is simpler than the C piece, there's no radiused nose, just the roller lead-in radius (which is more of an angle, really), then the locking angle flats that are ground, and then a "lead-out" that is a slight radius into the edge of the LP. It's only about 6mm thick, and the ground areas on the shank are about 8mm OD.

Also, how can I determine the angle needed for a safe delay, and a slightly longer than normal delay? I want to set the LP angles such that the gun barely unlocks reliably, then I'll lighten the locking arm spring slightly to get reliable, slightly less violent, extraction. Mainly looking for longer lock-up time as an accuracy aid, without going full roller-locked. I am thinking along the lines of 30 or 35 degrees, considering that the .308 HK21E is a 36 degree LP, and the 5.56 HK23E uses a 70 degree LP...the Cetme L uses a 50-53 degree according to Biffj, so that seems like a "half" change in the big gun, so a "little less than half-change" in the little gun says 30 degrees or so.

I might just have to build/buy a surface grinder to finish this one. that means building two tools for one gun...I'd have to make a LOT of barrels and LP's to make that effort. Maybe MarCoMar wants to build a few 6.8SPC Cetme Ls....or maybe they'll sell me a few more kits to play with.

holescreek
07-20-2018, 11:42 PM
Near the end of the locking piece type study (another thread on this site) there is a formula that another member posted that allows you to enter your known cartridge data to calculate the nominal LP angle. He came up with it when we were discussing future odd caliber builds. It should have everything you need. I suggest you not make weakening your springs part of the plan and work up the proper LP angle for your application. Doubly so if your plan is to make and sell parts.

Edit: Formula in post #198 here: http://www.militaryfirearm.com/Forum/showthread.php?17470-Locking-Piece-Type-Study/page20&highlight=locking+piece+type+study

Xnke
07-23-2018, 08:36 PM
New magazines showed up today from ASC. 10rd 6.8spc AR mags, they feel nice and load easily, and do not show any measurable bulging when loaded to 10rds. They slide into the magwell section of the flat as nice as anything, and I didn't have to knock off any burrs or rough spots on the feed lips. They look very close to the 12rnd original cetme L mag, too, which is nice.

Xnke
07-25-2018, 11:23 PM
Ok, so I have been working with the formula from the other thread. Best I can tell, the information is incomplete or just garbage, as no matter what numbers I put in, the locking piece ratios come out in double digits or higher.

LP = Pv * Pm * (Bd/Pd)^2 / Cm - R / Bv

First off, the formatting is pretty confusing. If I run any kind of numbers through the above, I get garbage out.

The other thread suggests that carrier mass be in pounds per ft-second squared, which seems wrong, then gives a conversion to grains, which also seems unnecessary. The velocity of the bolt is in feet per second, so I assume the projectile velocity is also feet per second.

Anyway:
Carrier mass is 490.83 grams, which includes the carrier, bolt head, rollers, roller retainer and pin, locking piece, firing pin and firing pin spring. The whole recipricating mass.

Projectile velocity is 2725fps at muzzle.

Projectile mass is 7.45 grams/115 grains

I converted grams to the funky pound/ft-second^2 units, but if I knew the metric constant for R I could skip all that and just use grams.

Assuming the formula is complete and actually gets worked as written (I suspect some parentheses are missing...) I get an LP ratio of 14.7. That ain't right...

holescreek
07-26-2018, 01:20 AM
The guy that wrote the formulas was a math wiz and I haven't seen him post anywhere in a long time. I trust his formula, but don't understand any of it.

Xnke
07-26-2018, 06:31 AM
Oh I am sure it was correct, the problem is that there are no units given, and the formula as written is very ambiguous as to the order of operations. When I get to a bigger keyboard, I will try to work it out again.

biffj
07-26-2018, 09:55 AM
I don't think you want to include the bolt head weight in your calculations. The carrier is accelerated by the locking piece while the bolt moves very little. The only effect the bolt weight will have is on momentum after things get moving. You want to keep bolt speeds down below 20fps. Above that ejection becomes a problem along with the battering you'll get on parts. Below about 13fps you won't have enough energy to fully cycle.

Frank

holescreek
07-26-2018, 10:58 AM
I came up with 61 degrees. So if the existing LP is 53~55 the rifle should run, just have a long cycle time.

Xnke
07-26-2018, 12:13 PM
Ok Holescreek, how did you come up with that number? I came up with a 5.5 ratio, but that was after removing the Bolt Head mass from the total carrier mass. (Bolt head, rollers, roller retainer, and retainer pin weigh 50.02 grams)

What I've got so far:

[Pv * Pm * (Bd/Pd)^2] / [(Cm - R) / Bv]

Where

Pv = 2725 FPS
Pm = 7.45 grams/115 grains
(Bd/Pd)^2 = 2.3209
Cm = 440.81 grams/6796.62 grains
R= 55
Bv = 8.3 feet per second (according to BiffJ maybe this should be something closer to 14 feet per second?)

It's got to be the masses that I've got wrong here. I'm coming up with a LP ratio of 5.5, which would point to a locking piece with less than a 30* angle to it. Your calculation came up with a locking piece requiring MORE angle, which would suggest that the 6.8 SPC would have less bolt face impulse than 5.56 NATO. It may, actually, but I'll have to check and see. I think bolt thrust is higher than 5.56, and muzzle energy is significantly higher, but the larger case head might factor in that?

I'll check back after lunch, got a big drawing to do here at work.

holescreek
07-26-2018, 02:08 PM
For some reason I had 300BO on the brain. Forgot about the 6.8 SPC aspect.

Xnke
07-26-2018, 02:34 PM
That's OK, but can you post your numbers and how you used the formula? I am still not getting answers that seem "realistic" yet. It would be nice to see how you've done it so I can double-check myself.

Xnke
07-27-2018, 10:12 AM
OK, so a buddy of mine and I worked this out last night.

The mass units do not matter...they get cancelled out. SO you can work in grains, grams, kg, pounds-weight, pounds-force, it doesn't matter as long as both are the same. The result is a velocity ratio. Also, the formula works directly left to right, somewhat ignoring the order of operations. I'm tinkering with an excel spreadsheet to calculate this stuff tonight.

Running the numbers for the Cetme L and 8.3fps bolt speed, I need a velocity ratio of 6.4. That's way lower than 30*, "off the scale" so to speak, but I am plotting a curve based on the data in the other thread to try and predict what angle is required to produce any desired velocity ratio. Gonna put this in the spreadsheet too, I think.

holescreek
07-27-2018, 01:56 PM
Just looking at the wikipedia data my WAG is 40~45 degrees.

Xnke
07-27-2018, 07:22 PM
More like 17 Degrees. Plotting the motion ratio data we have in the other thread, then applying an exponential delay curve to extend the data, the graph looks like this:

https://i.imgur.com/6YZRiru.png

Solving the equation y=10(0.9736)^x for X, which is our locking piece angle, when y is 6.4, X is 16.6 degrees.

In a plug-and-play arrangement, the formula to generate the locking piece angle from the motion ratio is:

X = (ln(Y/10)) / (ln(09736)) where ln is the natural logarithm.

Still working on getting the spreadsheet working.

scottz63
07-27-2018, 07:34 PM
This thread is making my head hurt! Lol! :)

holescreek
07-27-2018, 07:57 PM
Sorry, not buying it.

I run a 10 degree LP in my .243 Cetme purposely so the bolt will remain locked until the gas rod kicks it open. The pressure numbers I saw today in Wikipedia for your cartridge of choice aren't likely to provide the force needed to cycle, even with the lighter carrier. I guess we'll find out sooner or later.

Xnke
07-27-2018, 11:16 PM
I am not disagreeing about the angle being low. That's part of why I kept thinking I was working the formula wrong. The bolt velocity of 8.3fps seems low, to start with. What kind of bolt velocity *is* actually reasonable, I do not know.

You already know cartridge pressure is only part of bolt force equation, remember the case head is larger, and the projectile is heavier.

5.56 bolt thrust = 6833 pounds-force (case head diameter is .378")

6.8SPC bolt thrust = 7064 pounds-force (remember, case head diameter is .422" here.)

I'm thinking I should be looking at 40 degrees too, but I seriously think the Bv constant for bolt velocity is too low at 8.3, so I'm gonna work it again at BiffJ's suggestion of 13FPS to ensure extraction and ejection here.

[{(2725FPS * 7.45g * 2.3209) / (434.81g)} -55] / 13FPS = 4.10

Solving

X = (ln(4.1/10)) / (ln(09736))

X = 33.32 degrees. This seems more reasonable than 16.6 degrees, by far, but it also jives with my initial WAG of 30 degrees too much for comfort. I really think I need some benchmarks for bolt velocity to know where I should be looking.

EDIT: I have no idea where I came up with 6.4 as velocity ratio at 8.3fps...it was actually 5.5. Which works out to be 22 degrees, still very low. I think I'll start at 33 degrees, then if that is too slow of a bolt velocity it'll cycle but not fully...should either not eject, or weakly eject and fail to strip the next round. If 33 is too slow, I can always make a 37 or 40 degree LP...but if I start at 40, and it's not enough, I don't think I get a second chance.

holescreek
07-28-2018, 05:29 AM
It's the middle of the night and I woke up thinking about an old document I'd read. A quick search found it again. Check out pages 20 and 36, then compare the component weights.

http://www.dtic.mil/dtic/tr/fulltext/u2/849576.pdf

Xnke
07-28-2018, 01:28 PM
That's excellent information. Reading through that, it seems that the HK91/93 has a bolt speed between 20 and 25FPS, MUCH higher than what I've been looking at.

I guess I should work the math knowing the locking piece angle, and 5.56 cartridge information, and back-calculate the bolt velocity of a stock CETME L.

Xnke
07-29-2018, 12:52 PM
So did some more number crunching.

6.8SPC:
16.5 degree locking piece = 8.3fps bolt velocity
23.7 degrees = 10fps
40 degrees = 16fps
45 degrees = 18fps
50 degrees = 20fps
52 degrees = 22fps

5.56:

4.02 grams
2881fps
0.378 case head
2.873 = (Bd/Pd)^2

5.56:
50 degrees = 8.3fps
60 degrees = 10.fps

So looks like 8.3fps is roughly the factory bolt speed...crap.

holescreek
07-29-2018, 03:35 PM
So did some more number crunching.


So looks like 8.3fps is roughly the factory bolt speed...crap.

Then why are the bolt speed numbers so high in the PDF?

biffj
07-29-2018, 09:43 PM
I can tell you from actual testing using high speed video that the L bolt speeds when running properly are between 13 and 18 fps. When it gets up to 20+ fps you start having trouble with ejection and beating things up. At speeds below 13 fps reliability drops considerably.
Running at 22 fps the bolt carrier starts bottoming the springs and hitting the buttstock. Even with longer new made springs it can result in damage. I've seen one stock that broke at the wrist from bolt carrier impacts. All of these things resulted from setups that had either insufficient or excess bolt gaps and all used the same factory locking piece. At .018 bolt gap the travel and impact were as bad as with no bolt gap. At .006"- .014" the bolt stopped well short of full travel.
So I'm not sure where to go with the calculations but compare the bolt thrust calculations for the 6.8 and 223 to get a good idea of how much difference there is. I'd use that info as some sort of guide with more confidence than the more complex calcs.

Frank

Xnke
07-29-2018, 10:49 PM
Then why are the bolt speed numbers so high in the PDF?

Because they are HK91/93 numbers, and have different carrier weights and different spring rates, I'd guess.

I assume the resistance constant is different for the Cetme L, than for the HK stuff. I'm going to start with a 35 degree locking piece and go from there. That splits the difference from 30 degrees and 40 degrees, and should produce a safe firearm, even if it's too slow to cycle through correctly. I can always make a 40 degree piece later.

biffj
07-30-2018, 12:07 AM
The L is definitely different than the 33. We did measure one of the 33's though it had a green mountain barrel that didn't like brass cased ammo. It was still in the 18-20 fps bolt carrier speed range depending on ammo. The carrier used has the tungsten granules and the spring buffer. No buffer in the stock.
35 deg sounds pretty steep to me unless you're loading the 6.8 pretty hot. Its not that much more bolt thrust compared to 223 as far as I can see. I'd think more like 40-45 deg would be a better place to start. This is just empirical guesswork on my part of course based on working with the different variations of the roller lockers and their locking pieces. Chamber pressure vs area of the cartridge case its working on gives bolt thrust. Its not as precise and mathematical but might make life easier.

Frank

Xnke
07-30-2018, 01:49 PM
That's true, it's only an extra thousand pounds of force. (Just a thousand pounds...)

It's not too hard to make these things. I've got a few pieces of O1 drill rod in the shop, I'll try and spin one up tonight.

biffj
07-30-2018, 08:46 PM
Keep us posted on progress..... I made a couple with O-1 and they worked well. My guess is 43 deg will do it.....
Frank

Xnke
07-31-2018, 12:06 AM
How do you figure on 43? Just out of curiosity, that is.

I'm half tempted to spin the angle on 'em, then work the sides with a flat file till they make the lockup. I've got four 0 degree locking pieces that are in the heat treat oven now. Should come up about 56RC in the morning.

biffj
07-31-2018, 01:44 AM
Basically its a wild ass guess on my part. I tried 47 deg with the 223 and it worked but not reliably. It was a little too much for the round and the barrel length.
I made up a jig for grinding. I turned the locking piece with an angle that was a little flatter than milled the flat sides with a few thousandths for grinding after heat treat. After milling the flats I set it up and milled the lug. At that point I sent it to heat treat and had it drawn back to about Rockwell C 48. Avoid sharp edges and radius any grooves. Once back from heat treat I put them in the spin fixture to grind the flats to thickness evenly on both sides. Next I put the thing in my grinding jig which indexes each side by the lug and base of the main body. It allows grinding of the angle I want and I can measure to the jig as a reference on a surface table. It makes holding things a lot easier. It does require a different jig for each angle unless you've got a sine table or something to allow that.
Holescreek had some really good pictures posted on one of the threads showing how he made up some too and it was very helpful.

Hope something there helps. I'd post some pics of the jig but haven't figured out how.....

Frank

holescreek
07-31-2018, 10:28 AM
The toughest and last step in making a locking piece is the same one that scraps it out. It's getting the proper distance from the roller contact surface on the shoulder angle to the skirt that rests against the carrier face. You can use whatever method you want to grind the angles (except "spinning" them on?) but if the shoulder to skirt distance is wrong the LP becomes just another shiny object.

I use my setup to measure the length of a good LP and then grind my new LP to an equal distance. The grinding has to be done on the angle-side of the LP because A) the skirt is your reference surface and B) the skirt to locking tab distance is already set.

If you come up with an easier way, please post photos!

Xnke
07-31-2018, 10:35 AM
So you have made Cetme L locking pieces, BiffJ? I just checked the hardness of my three today, they average RC51. (all cetme L LP's.) When I get home, I'll retemper to bring the hardness down a bit to match, right now the ones I made last night should be about RC56.

I milled mine to thickness in the flat area, as all three LP's I measured were slightly different, and none of them were ground on any surfaces. Even the round areas were as-turned. In fact, of the three I have the only dimensions that match are the angle of the faces, and the diameter of the firing pin hole.

By spinning, I meant I'd turn the angle in the lathe, then finish grind after hardening, to reduce the amount of grinding that had to be done. I didn't do it on these first four because I have access to a CNC surface grinder at the moment.

Xnke
08-05-2018, 10:38 AM
So in my plans is to produce a US made stock set for my rifle. I could just buy from MarColMar, but I have no idea when they could/would/might be available, and I doubt they would tool up to make the handguard I would like to have-the early style square, full-length handguard.

In fact, the top left section of the first photo is a very fair representation of the gun I am going to be building.

I am looking for one of these handguards:

https://i.imgur.com/TPF73OK.png

https://i.imgur.com/FCmMGt1.jpg

If anyone here has one they'd be willing to sell or loan for me to make a mold from, I'd be very interested in buying or borrowing it.

biffj
08-05-2018, 01:05 PM
That handguard you'll probably have to make yourself. After looking through about 12,000 kits I've yet to see any of the early sets in any of the kits. Its possible but not likely that there are any. Marcolmar is waiting on color test material for the green and tan furniture to get started on production. I don't think it will be long before new furniture is available. The butt, pistol grip and buttpad will be the same for early or late type and the buttpad will fit the original stocks as well. It doesn't help for the 922(r) count but some of the buttpads are pretty poor in the kits and the new ones are nice.
I don't think the original long square handguards have much room for air to flow around the barrel and that could be one reason they dropped them early. That can be a problem for the plastic and without enough cooling air flow the barrels would overheat much easier too. I prefer the look of the early handguard myself but don't think its very practical for shooting. I could be wrong too....haven't been able to find one to check it out.

On the locking pieces the dimension of the flat are not critical. It is important to keep the firing pin centered but mechanically its not critical for operation of the rollers. Cutting the angle for the rollers on the lathe is a really bad idea. The originals were ground to insure they're flat. If you cut them on the lathe they'll be rounded and that reduces the area in contact with the roller. This will cause an increase in the pressure on the surface in contact and will peen or dent it. That will increase the area in contact by beating it down but that is not helpful. It can also damage the rollers because the point in contact is now tiny rather than being across the whole roller. If the locking pieces you have are not ground on that surface they aren't right. Post a couple pictures if you can...I haven't figured out how.

Frank

Xnke
08-05-2018, 02:36 PM
The point of turning the angles on the lathe is to reduce the amount of material that has to be ground off, and lessen the time and wear on the surface grinder-not to establish the locking faces. I am aware of how the point contact and line contact will differ in this application, no worries there. Taking 3-4mm off the sides with a surface grinder at 0.01mm per pass is going to take MUCH longer than taking 0.5mm off at 0.01mm per pass.

To post photos, Frank, upload them as an unlisted photo to IMGUR.com. You'll need to create an account, but as long as they are unlisted, they won't be posted for public search. Once they are hosted, when you click on them in your photo album, it will bring up a menu that has an option called "BBCode Image" and that's the line you can copy and paste directly into you forum post here.

holescreek
08-05-2018, 05:59 PM
Post a couple pictures if you can...I haven't figured out how.

Frank


To post photos without an outside photo storage account click on the "go advanced" button in the bottom right corner of the reply screen. Type in your text then scroll down to the " manage attachments" button. When you press it a small window will pop up click on the "add photos" button then the "browse" button. Clicking on the browse button will allow you to choose photos from your computer by opening the photo (BIG NOTE: This forum only allows photos of 100kb or less - more about that later). Once you've opened the photo the browser will return you to the selection screen. You can add more photos by choosing the + sign and browsing again. Once your done adding photos select "upload" and thumbnails of your photos will appear. Uncheck all but the photo you want to place in the text then click on the insert in line 1 button. The software will add a code into your text containing the photo.

It's a real PITA being bound by 100kb photos ever since digital cameras started taking 8mb pictures. To reduce a photo you need to open it in microsoft paint and click on "resize". I change the number of pixels in one axis and hit OK (the other axis will coordinate with the axis chosen). Sometimes I have to reduce a photo several times to get it below 100kb. A good starting point is around 800 pixels in the X axis. once you've reduced it press "save" then select "properties" to see what the size of the photo is.

Sampedro
08-06-2018, 02:53 PM
The first picture you mention, has the drum sight of the Infanteria de Marina CETMEs. Only them used it.

About the square hand-guard, the Army ditched it pretty early (none entered service before change to round half-lenght handguard), so I doubt any is available in the market, since all demilled stocks seem to be ex-Army or ex-Air Force.
A few may be left in some naval armory (since our marines used them for some years), but most of them went to law enforcement (mostly Guardia Civil) and are still "in use" (whenever they are allowed to use them).

I'll ask around to see, but my company still has a coupl dozens and it's the round handguard.

Xnke
08-06-2018, 03:13 PM
Sampedro, have you ever had the opportunity/misfortune to handle one of the early weapons? It's looking like I'll have to replicate the handguard from scratch, so I am looking for things like taper, wide at bottom/narrow near charging tube, etc. I'll be casting it in a high temp/high rigidity plastic that's mixed with expanded mica flake to increase rigidity and thermal properties. Forming the sheetmetal will be relatively simple, but it's the plastics that I am as yet unfamiliar with.

I thought somewhere on this site was a demonstration thread of how one of our members was casting resin replicas of his carved pistol grips for the CETME C, but I can't find it lately. I recall it being very clear and detailed, too!

Xnke
08-06-2018, 03:13 PM
Sampedro, have you ever had the opportunity/misfortune to handle one of the early weapons? It's looking like I'll have to replicate the handguard from scratch, so I am looking for things like taper, wide at bottom/narrow near charging tube, etc. I'll be casting it in a high temp/high rigidity plastic that's mixed with expanded mica flake to increase rigidity and thermal properties. Forming the sheetmetal will be relatively simple, but it's the plastics that I am as yet unfamiliar with.

I thought somewhere on this site was a demonstration thread of how one of our members was casting resin replicas of his carved pistol grips for the CETME C, but I can't find it lately. I recall it being very clear and detailed, too!

biffj
08-06-2018, 07:00 PM
This is just a quick test of the pic posting. I left my phone at my buddies shop this afternoon and thats where the pics of the jig are. I'll get them copied to the computer tomorrow and posted.

The pic shows two of the locking pieces I made up with a 47deg angle. The original is in the middle. The ones I made look a little funny in the pic but its due to the camera.

Frank

holescreek
08-06-2018, 09:18 PM
This is just a quick test of the pic posting. I left my phone at my buddies shop this afternoon and thats where the pics of the jig are. I'll get them copied to the computer tomorrow and posted.

The pic shows two of the locking pieces I made up with a 47deg angle. The original is in the middle. The ones I made look a little funny in the pic but its due to the camera.

Frank

Frank,

Have you posted that photo anywhere else? I can't shake the feeling that I've seen it before. I even started to comment on the missing roller relief grooves but thought I already had. BTW, you commented you weren't far from Dayton, where are you located?

biffj
08-06-2018, 10:36 PM
I might have posted that one weapons guild but I don't remember doing so. You're right, the roller relief grooves weren't in those 2. The shop is in Richmond Indiana. I'm a little west of there at home but not much.

Frank

holescreek
08-06-2018, 10:55 PM
I didn't realize that Marcolmar was so close. Seems like there's a lot of roller locked research going on, I'm a little curious as to the equipment you used to measure bolt speed. I'm still trying to figure out a cheap way to measure recoil force that doesn't include lugging a bunch of equipment to the range.

Edit: I just remembered where I saw the pic, it was in my "how to make a cetme LP" thread. Just part of getting old I guess.

Sampedro
08-07-2018, 10:18 AM
Sampedro, have you ever had the opportunity/misfortune to handle one of the early weapons? It's looking like I'll have to replicate the handguard from scratch, so I am looking for things like taper, wide at bottom/narrow near charging tube, etc. I'll be casting it in a high temp/high rigidity plastic that's mixed with expanded mica flake to increase rigidity and thermal properties. Forming the sheetmetal will be relatively simple, but it's the plastics that I am as yet unfamiliar with.

I thought somewhere on this site was a demonstration thread of how one of our members was casting resin replicas of his carved pistol grips for the CETME C, but I can't find it lately. I recall it being very clear and detailed, too!

The older square handguard ones, just in a range session with a Guardia Civil LC. But just a mag at the range doesn't qualify me for saying much. They just showed us a couple melted handguards, and warned us not to shoot full auto nor more than a couple magazines in semi very quickly.

"Regular" Army L is what I have more experience, of course.
The ones made before 1992 (some say before 1994) were quite reliable, and even got a barrel red hot in a four mags full-auto dump (summer in San Gregorio, so probably around 40 celsius), but no problems at all, and the handguard plastic was unaffected, despite you could barely hold the rifle.
Then came the "expense-cutting measures" (a regular L cost the country 150.000 pesetas in 1995, around 1200 US$), and all the rifles I had assigned made after 1995 were junk, even right out of the factory. Weak springs (hammer and main/recoil spring) were the most common problem, with magazines out of spec (and sometimes mag-wells too) and ammo problems (very hard primers, combined with the weak hammer spring) coming second. Using NATO magazines (I had a US-made one found in a maneuver field in Sardinia) solved many feeding problems.
Curious thing is LCs made after 1995 didn't give so many problems (except for the ammo and magazine issues), probably due to different main spring and different shape of the bolt.

biffj
08-07-2018, 11:45 AM
I didn't realize that Marcolmar was so close. Seems like there's a lot of roller locked research going on, I'm a little curious as to the equipment you used to measure bolt speed. I'm still trying to figure out a cheap way to measure recoil force that doesn't include lugging a bunch of equipment to the range.

Edit: I just remembered where I saw the pic, it was in my "how to make a cetme LP" thread. Just part of getting old I guess.

It is tough getting old....but better than the alternative.

We measured bolt speed by using a high speed camera to take video of the guns running. I built up a test gun and marked the bolt and the receiver with hash marks for reference. I think the frame rate was 8000fps. By counting frames and travel its possible to calculate the speeds. We tried all sorts of different setups with bolt gap, ammo, springs as well as running the LC and an HK33. At some point we're going to rent a color camera and go through the tests again. The cameras are really expensive but can be rented. There is a shop in Indy that has a pretty wide variety of equipment to do this sort of thing. The cameras are easy to run and come with the computer and software to do what we needed. We tried some video of bullets coming out the ends of the barrel but 223 is fast and we didn't have enough light or experience with the system to get very good video. We did get some neat shots of my 510 whisper suppressed and unsuppressed shooting 50 cal tracers at 1050 fps.

Frank

Xnke
08-07-2018, 10:03 PM
The fun part is neither Holescreek, Biffj, or MarColMar are far from me-you are all between my house and my little sister's place.

biffj
08-07-2018, 11:51 PM
I seem to be having a senior moment here....picked up my phone this afternoon and can't find any of the pics I took of the locking piece jig on the surface grinder or even just the jig. Can't find them in my computer folders and I know I took them.....I'll have to take some of the jig and just explain. Sorry I can't find the ones of it in use.

Frank

biffj
08-08-2018, 11:29 PM
Hopefully these pics will make sense. The first is my grinding jig with an L locking piece in place. I set up the surface grinder with the jig oriented so the locking piece was ground lengthwise. Probably doesn't make any difference functionally but its easier to grind when you're not trying to pop the thing off the jig. I used a small Kanttwist clamp to hold the locking piece to the jig. The grindstone pushed the locking piece against the stop pin in the jig.

The second picture shows my test locking piece in place. This one was only used for measuring the angles. We used a vernier protractor to measure it because we don't have an optical comparator and didn't have the CMM at that time.

Frank

holescreek
08-08-2018, 11:41 PM
Way more complicated than mine! :084:

54462

54463

BTW, If you have small parts that need precision measurements I sit (freezing) in a QC lab every night with about $2M worth of measuring equipment.

Xnke
08-09-2018, 06:46 AM
Ha! I am a CAD draftsman...who's office is in the QC/metrology lab, which stays 68*F 24 hours a day. I sit right between the CMM and the metrology tool cabinet.

biffj
08-09-2018, 10:13 AM
I see the designs are very similar....good to know I'm not way out there.
I only freeze in the grinding room. I have to use the grinder at a friends machineshop and they do a lot of precision stuff...68 deg all the time. My shop is a pleasant 77 deg.

Frank

holescreek
08-09-2018, 03:18 PM
Last night when I was typing the response it was 66.2 and I was wearing a sweatshirt. It's supposed to stay 68-70 but the humidity control goes nuts for a couple hours every night.

Frank, I'm glad you posted the pic, I was imagining something completely different from your description. In fact, if I was going to make another style of fixture I might try one like I was imagining!

As an amateur CAD guy I've found it useful to do a layout of a new locking piece angle in place with the shoulder angles contacting the rollers in the trunnion and a .016" bolt gap between the bolt head and carrier. Knowing the length on paper doesn't usually work out the same as measuring in place because of stack up tolerances, but gives me a target for machining.

biffj
08-09-2018, 07:39 PM
Frank, I'm glad you posted the pic, I was imagining something completely different from your description. In fact, if I was going to make another style of fixture I might try one like I was imagining!

Ha ha! Yep.....this is why I work with machinery. Not a people person who can transmit info verbally very well....not so hip with computer info transfer either but I try. Pics make up for a whole lot of that.
I am still using autocad 2000 but I try to set up drawings to understand what's going on too. The L is a bit of an oddball though and doesn't follow all the logic of the HK and Cetme C roller lockers. We're coming to understand it a lot better now though.

Frank

holescreek
08-09-2018, 09:04 PM
The idea I was thinking about was buying one of the cheap adjustable angle blocks like the pic below and mounting a fixture on top of it to hold the LP. Even if you didn't trust the scale on the side you could verify the angle with a sine bar before grinding.

54469

biffj
08-10-2018, 12:42 AM
That is an excellent idea. I don't do a lot of surface grinding so I'm always thinking it has to be something that the magnetic chuck will hold....keep forgetting you can put a tool makers vise or other stuff on the thing too. Adjustable angles are better than cutting a new jig for every angle....

Frank

rebel49
08-10-2018, 12:57 AM
Mike those little angle blocks are pretty accurate to the scale, in minutes of angle anyway. I've had one for years and used it for grinding angles. Go to seconds of angle, then a sine plate/bar is necessary.

holescreek
08-10-2018, 01:05 AM
We learned in the LP study that the original LP's weren't all that close sometimes, even +/- a degree would be close enough for most shooters. Once you got the fixture made for mounting it'd be easy to grind it in to the zero setting on the protractor. I wish Frank would have put the idea in my head a few years ago.

Xnke
08-10-2018, 08:46 AM
Hopefully Monday I'll have some time on the cylindrical grinder to do the shanks, I pretty well have one of the old manual surface grinders to my self anymore-no one wants to use it because it doesn't have coolant fitted.

Drew the LP blanks back to 50RC, when I tested the rollers in my kit they were 56RC so I want to maintain a difference of about 5RC in hardness-this prevents galling and erosion pretty well. Next on the list of stuff to make is the 40 and 45 degree grinding jigs.

On these LP's, the angle is measured from the side of the LP, angling in toward the firing pin clearance hole, correct?

biffj
08-10-2018, 11:38 AM
From my measurements its the included angle between to two ground portions in contact with the rollers. I set my jig up with half that angle or 23.5 deg to grind the locking parts for my 47 deg unit. The sides on many of the locking pieces are not necessarily very precise as they don't really do a lot. Many of the locking pieces have wear on one side and not the other because they aren't concentric to the centerline. I've also seen a few where the sides are not even parallel to the centerline. They don't have any real function other than to keep the bolt head from flopping around too much. The ground angles need to be concentric to the centerline of the locking piece for which I use the shank as a reference.
Does that make sense?

Frank

Xnke
08-10-2018, 01:12 PM
Yes, that clarifies quite nicely. May get the chance this afternoon to put the part on the cylindrical grinder and get the shank ground, but the heat treat schedule I am running today (CAD, inspection, assembly, reverse engineer, and now hear treat....) is pretty tight.

64bonneville120r
08-12-2018, 07:30 PM
Hello , new member here.
I've read most of the info on this site about the cetme / hk . I've an idiotic idea and you guys seem to be my
only resource for it. I'm not trying to hijack, but one of u has a 50 whisper. I am planning to build a version on a cetme platform. Mite ir be possible
to ask if you could make a lp for me? I plan to make 2 actually, one on a 98, to work out loads, ie an m24, the other a cetme. If my intrusion irritates, please disregard but do carryon and have a great day. Thank you.

biffj
08-12-2018, 09:39 PM
I build barrels for a couple different rifles, chamber and load the 12.7X48 which is an equivalent to the 510 whisper and shoot as much of it as I can. The 50 whisper is a completely different round based on the 460 weatherby case and it would make life even tougher. I guess I have a lot of experience with it and I'll say flat out the Cetme isn't a candidate for this round either in the C or especially the L. The case head diameter is .585" compared to the .473 of the 308 and .378 in the .223 in the L. I think the C might have a variety of different calibers as choices but if you go larger than the .308 case head it starts becoming a problem. The same is even truer for the L which has very little extra space. I think for the L the 6.8 SPC is about as big as you can reasonably go. Trying to fit anything in the mags becomes a problem as well.

I love fiddling with oddball stuff but no way I'd think about the 510 in any of the roller locks. No idea what locking piece you'd need.

I saw your other post concerning a swap barrel setup and while I haven't built one yet I've got drawings for a roller locker intended to use a swap barrel. I don't think it will be too tough and HK has already done it with the model 21/23. Barrel change in belt fed machineguns is important and with the 21 you can swap out the bolt head, feed mech and barrel to go to .223.

Frank

Xnke
08-12-2018, 10:15 PM
Can confirm, you're not gonna go bigger than 0.422 casehead diameter in the Cetme-L. I'm right on the edge of breaking through the bottom corner of the bolt. 7.62x39 casehead won't fit, otherwise I would have built this in 6.5 grendel or 7.62x39. Heck, 7.62x39 barrels can be had, which would make it much easier than the 6.8SPC is going to be.

Updates on my build, I have lapped the green mountain barrel blank and have drilled 10 dummy barrels to practice fluting the chambers on. I have built the ECM tooling to ECM the chamber, and doing a test chamber has taken about two hours, and the machine got HOT. I don't have enough machine here to do the whole chamber effectively, so I'd have to ream the chamber, at least a short chamber, then ECM the rest of it. I have not come up with a clever way to do all the flutes in one go, yet, but hand-scraping them in there works fine.

Last night, I stripped, scrubbed, and modified the trigger pack to accept the full-auto denial parts, and welded the denial bar into the receiver flat. Going through the trigger, there just isn't any reason for the trigger pull to be so heavy-almost 14lbs in mine! Removing the trigger spring and just operating it with the mainspring and sear spring, trigger pull was less than 6 ounces. I cannot find a source for more trigger springs, so I haven't modified mine yet. I already had to make a new trigger spring pin as the originals are riveted into the trigger, and have to be drilled out.

Guess I am looking for a different trigger spring to swap in, as I don't want to modify my only original.

64bonneville120r
08-13-2018, 08:28 PM
This thread and the many before it that get into the realy nitty gritty are inspiring! The cartridge I'm lookin at to use is the 50ae case straight w .510 bullets either jacketed or lead. Head all bit fits the Std bolt face for 308. And does feed with little adjustment in a bolt, been playing w m14 mag for a modified 98 platform ie archangel. Works surprisingly well. Oal is limiting. Id like any feedback on the change barrel. Thank you all for sharing so much time , trouble and research with the rest of us . Thank you all in advance for the help I've found here just in the info!

holescreek
08-13-2018, 08:46 PM
The HK/Cetme is a roller delayed blowback rifle that uses chamber flutes to allow gasses to pass between the brass and cartridge to keep the cartridge from sticking in the chamber. If the cartridge doesn't move neither does the bolt. Even if you build the rifle on an HK platform where there are locking pieces available you "may" still need to flute the barrel chamber. I say "may" because chamber flutes aren't always required on straight wall cases. If you get that far, you get to be the guinea pig.

Xnke
08-13-2018, 11:31 PM
Set up block for the grinding is on the docket for tomorrow evening. Plans are to finish the locking piece this week, then get started on the barrel machine work. Assuming I can get the ECM tooling sorted and working, I should have a shootable prototype by December.

Now, the problem with that is keeping the 922R parts flowing! Currently the following are in play:

Magazine Body
Magazine Follower
Magazine floorplate
Barrel
Receiver
Sear

That's 6 out of 7 required-I always like to stay under then 10 required parts, so I will do the following as well:

Buttstock
Pistol Grip
Forearm/Handgrip

and since the barrel/bolthead/magazines are all caliber specific, they can't be swapped for imported parts. The muzzle attachment is also a contender, as it'll need to be threaded for a non-metric barrel and sized for .27 caliber, so that tends to rule out imported parts, too. That gets me down to 10 imported parts, which is legal even if I don't modify the forearm, stock, and barrel.

holescreek
08-14-2018, 01:01 AM
No muzzle device?

http://thegunwiki.com/Gunwiki/BuildG3VerifyCompliance

biffj
08-14-2018, 01:05 AM
This thread and the many before it that get into the realy nitty gritty are inspiring! The cartridge I'm lookin at to use is the 50ae case straight w .510 bullets either jacketed or lead. Head all bit fits the Std bolt face for 308. And does feed with little adjustment in a bolt, been playing w m14 mag for a modified 98 platform ie archangel. Works surprisingly well. Oal is limiting. Id like any feedback on the change barrel. Thank you all for sharing so much time , trouble and research with the rest of us . Thank you all in advance for the help I've found here just in the info!


I know of a few guys shooting the 50 AE as a subsonic rifle round but its a .500 bore not .510. You can blow the case out to fit the .510 stuff but it looks really funny and I'm not sure how safe it is. The rim is rebated to the same diameter as the 44 mag so fitting the 308 bolt is possible. Not a lot of room for powder so heavier bullets are a bit iffy too. I run about 28 gr of AA1680 in the 12.7X48...I can get about 35 in the case with a 650 gr BMG bullet before things go wonky. If you run 500 diameter bullets there is a pretty good selection up to about 550 gr and they would make great subsonics.

Xnke I hope your efforts go well. This could be an interesting project. Making the muzzle attachment is pretty easy. I've done a few for various rifles to make up US parts counts. Its probably the easiest bit you'll do.

Frank

Xnke
08-14-2018, 11:59 PM
Problem with counting your muzzle device is just like counting a magazine for a Cetme C-then you can't use the wild abundance of import stuff. But in this case, there is no import muzzle device that will fit and function so counting it seems like a no brainer. The big question is then what happens when you take the muzzle device off? Is that not a 922R violation?

holescreek
08-15-2018, 12:47 AM
No, if there is none it cant be counted. You're far more likely to use a different magazine.

Xnke
08-15-2018, 03:17 PM
That makes good sense. I'll definitely be making some kind of muzzle device, flash hider or similar.

How safe is drilling a .223 M16A2 style flash hider to a size appropriate for .270? It appears to have plenty of meat in there to do the job, but before I drill it I'd like to be sure it's "OK" to do so.

biffj
08-18-2018, 02:07 PM
I don't think boring out the 223 flash hider to fit the 6.8 (.270) bullets will hurt one bit. Not a lot of difference in size.

Just thought I'd post these pics of an L barrel that I sectioned. It shows the wall thickness between the chamber and retaining pin as well as the same for the front sight base pin. Not a lot of room there. The thickness in the chamber area is right at .1" with just a little more at the front sight base. Imagine what happens when you increase the diameter of the chamber for a larger cartridge and the bore for a larger diameter bullet. These two spots get even thinner so your safety margins drop. I don't think the 6.8SPC is a problem but for others who want to set the rifles up for stuff like the 7.62X39 and others this should show why its not a good idea.

Also found that the barrels on the L are pretty thin overall. Makes them nice and light but they can't handle a lot of heat. Thin means they should cool pretty quickly but it also means they don't have much of a capacity to absorb much heat. Machinegun barrels are typically heavier mainly for heat reasons. These rifles seem to be pretty accurate but if you go dumping mags through they won't last long. Heat is the biggest killer when it comes to bore life. Keep that in mind when shooting them.

Frank

Sampedro
08-18-2018, 03:42 PM
...

Also found that the barrels on the L are pretty thin overall. Makes them nice and light but they can't handle a lot of heat. Thin means they should cool pretty quickly but it also means they don't have much of a capacity to absorb much heat. Machinegun barrels are typically heavier mainly for heat reasons. These rifles seem to be pretty accurate but if you go dumping mags through they won't last long. Heat is the biggest killer when it comes to bore life. Keep that in mind when shooting them.

Frank
Could get the area just before the flash hider red hot with a 3-4 mags-dump in full auto. Never happenned to me with a LC, but I didn't carry one for long enough.
Cannot talk about bore life, since the forced conscripts didn't shoot that much. Usually, rifles were retired after some kind of receiver or bolt carrier failure. More rarely, before something serious happenned.


Next week, my company has to test our battalion L's (from the last batches built, I think) to ready them for "Patrullas de Tiro" (military competition mixing long distance running, short sprints, and shooting), so I'll try to check a few things and tae pictures, IF I'm allowed close enough (I'll bring a few GI mags as a bribe, since they work better) ;-)

biffj
08-18-2018, 07:45 PM
Normally the barrel heats first from the area just in front of the chamber. Thats where the temps from the burning powder are highest. The heat increases forward from there. The chamber is usually the coolest part. Of course that is a relative term...
I would guess the LC wouldn't take long to start glowing up near the flash hider. There was a video that Doctor Phil Dater at Gemtech did some years back explaining how much heat was increased in an M16 by each round. Its surprising how quickly the barrel heats up to 700-800 deg F. At 800F you can see a dull red glow in sunlight. With the very thin barrels of the model L series there isn't much heat sink material to damp the rise in temps so it will heat quickly.

I've been learning a lot about the model L and I think a lot of the problems the guns had were due to improper setup of the bolt and carrier. Bolt gap is really important on these and a little big or a little too small makes a huge difference in bolt velocity. When it gets too high things start getting damaged. From what we've seen ideal is from about .006" to .015" with .010" being about ideal. Sorry Sampedro, I don't speak metric for sizes or temps. I know 1mm is .03937 inches.....
have a look at whatever rifle you're using and see if you can tell how large the bolt gap is.

Frank

Sampedro
08-18-2018, 08:30 PM
It was early dusk, from what I can remember, and couldn't see under the handguard (I wore combat gloves almost always in this kind of exercises, and it felt hot, anyway), so that's probably why the first red I saw was so forward. With MG42, it was usually the middle of the barrel that heated first, and that's the reason it stuck to my mind.

I don't know if I can get a set of gauges to chek it in time, but I'll try to at least get photos.
Actually, I never saw bolt gap checked at all, and started learning about what it was with my first (civilian) CETME C and the early internet. Maybe at battalion-level armorer they do, but not at company level.

But the ammo used now is different than the one used in the G36 and, out of personal curiosity, I'll try to find out if it's the older (as in made in the XXth century) Santa Barbara stuff, or just modern ammo loaded to different specs. Using the older ammo in gas-operated rifles was strictly forbidden after the start of the trials eventually won by the G36 (all submitted rifles were gas operated). NATO "forbid" it's use in 1994 during the deployment in Bosnia, but French, Italian and US ammo worked fine in the CETME L.

Xnke
08-20-2018, 03:39 PM
BiffJ, could you measure that barrel and give me the dimensions of that lip on the chamber end? For some reason I think it's 0.110" but I am not sure why I think that, or if it's correct at all.

Sampedro
08-23-2018, 01:09 PM
Couldn't get any measurement, only a couple pictures, and next week I'm on leave.
I'll try to get correct measures in early september, before we start the "back to reality" field maneuvers after summer.
https://drive.google.com/file/d/0B_hO0stbeMsFd214VGtwdHREVS0zb0xuUFhwcWNxNW5VS0Rj/view?usp=sharing
Sorry, couldn't share the picture properly. See in link (and let me know if there is any problem).

This particular rifle is in the 101-thousand range, so it belongs to the "bad runs". Despite some parts already substituted, main spring was givin a lot of FTF, not having enough strength to strip a round from the magazine if more than 10-12 rounds were loaded.
STANAG mags worked somewhat better, but the rules for the competition where they will be used only allow issued mags. Pity.

Ammo used was STANAG rated IMI FMJ (someone "forgot" the Santa Barbara ammo back at the base), with quite softer primers than old Santa Barbara-made ammo (so no light primer strikes), but in a couple of the rifles in the 63-thousand series (with the older open three-pronged flash hider, and main spring in better condition) the bolt didn't cycle back enough distance to strip a new round from the magazine.
The rest of the rifles of the 63000 series in my company had already been used as donors for some hammer springs, so it added to avoid light strikes on primers.

biffj
08-23-2018, 03:00 PM
I don't have a drawing of the barrel here but I'll see if I can get the thickness of the flange for you Xnke.

Thanks for the info Sampedro.....see what you can find out.

Frank

MarColMar Firearms
08-24-2018, 08:57 AM
Sampedro,

If an armorer's manual or CETME L manuals are floating about over there, we would LOVE to add one to our knowledge base...
We are building semi CETME L's here in the USA... please let me know!

Dave Bane
Owner
MarColMar Firearms LLC

Sampedro
08-24-2018, 01:39 PM
I will check, but since it's not in widespread use anymore, only brigade level, depots or arsenals would have them.

But now I'm on leave for a week ;-)

Xnke
08-28-2018, 09:55 PM
I would REALLY like to think that barrel lip should have been 0.1905"-ish, but that would leave zero bolt-face-to-barrel clearance. 0.170" would have been what I would expect-that would leave 20 thou clearance.

Unfortunately, I made an error when making the new locking piece. I'll have to make another one, but even so-this one is totally usable. The problem is that the nose of the locking piece was hitting the inside of the bolt head, which gave me a false reading during grinding the locking angles. I didn't catch it until it was 0.017" too late.

I could totally use this part-I'd just have to set the barrel lip at 0.2075" instead. And if a 40* LP isn't right, I'd have to make another LP with the same 0.017" undersize.

I made this for my grinding jig. You can see the black marker and punchmark where I drilled and tapped 1/4-20 Left-hand, then counterbored for an oversized, offset-ground washer. The screw and washer clamp the LP to the block, and because it's a left-handed thread, the rotation of the bolt forces the LP down and back against the dowel pin for accurate removal and replacement. It worked very well, so well infact that I never measured the slightest imbalance in the grinding, until two grindings in a row with zero change in bolt head location.

https://i.imgur.com/A3fYh07.jpg

biffj
08-28-2018, 10:53 PM
That jig looks good. I'm sorry I haven't been in the shop for a bit....visitors in from away. I'll be back in the shop thurs or friday and I'll get you a thickness for the flange. You can increase the thickness to increase bolt gap within reason. You can also shim the barrel if you're ready to push it out and then back in. Its a tough job to do but possible.
For a bit of help....its a good idea to cut out the notches for the rollers to clear the front end of the locking piece before you grind to the dimensions you want. If you don't you may grind further than you really want. The bolt won't go into the trunnion unless the rollers can sit completely flush with the sides of the bolt. You have to allow clearance and if you look close at the front of the original locking piece there is clearance cuts on each side to allow that. The rollers only touch in a very small area on the locking piece so anything further forward is not necessary. All this is within reason and you will see what I'm talking about when you go to put the whole package together. You can do all your test fitting with the rollers loose.....you don't need to pin the retainer in place and I don't even put the retainer in when I'm checking fit. It just keeps the rollers from falling out when you pull the bolt out.
Keep at it......
Frank

Xnke
08-29-2018, 08:26 AM
Yep, found that out. The big thing is the clearance notches are needed to allow the LP to go forward far enough in the bolt head to start with-without the cuts, the front of the LP will hit the inside of the bolt head about 0.020" from being in full "battery". That's what got me.

Setting the barrel back 0.017" is totally doable, and with the modified bolt face, this gun will never be in a position to use a factory LP. (Although there is plenty of meat on the factory LP to regrind to 40* with that 0.017" setback!)

holescreek
08-29-2018, 10:17 AM
I imagine that after there are a couple thousand Cetme L's in private hands someone will start offering custom locking pieces for them. Undoubtedly there will be individuals that want to shoot suppressed and not beat their rifle to death. I would hesitate to make a major change to any rifle now (such as you're describing with barrel setback) to save an hour's work making another LP blank and end up with a rifle that can't use stock parts later.

If there were Cetme L LP's around I'd pick one up to blueprint and put it in the LP type study here for public use. However, the L is such a rarity in it's own right there aren't even spare parts available commercially.

Xnke
08-29-2018, 02:08 PM
I'll gladly send you one, I have a few here. I'm a long way from setting the barrel anyway, so the plan is to make as many as it takes to get it correct. It's about two hours for me to make a blank, then heat treating has to wait til we get a batch of steel to run. I made this last one out of A2-it's got a little better wear resistance than O1, and I have a bar that's already right to the size I want for the front section-I am not sure why I have a bar that's 15.75mm OD, but it's close enough to fit the collet and still not take much work to get down to size.

biffj
08-29-2018, 10:15 PM
If there were Cetme L LP's around I'd pick one up to blueprint and put it in the LP type study here for public use. However, the L is such a rarity in it's own right there aren't even spare parts available commercially.


There are going to be a lot of them around in the next few years. There is a plan to at least do a 300 blackout version though it is unlikely it will handle both subsonic and supersonic ammo reliably. That will mean at least 2 different locking pieces.

Frank

Xnke
08-30-2018, 09:19 AM
HMG's 300blk did subsonic, supersonic, and suppressed with the factory locking piece, and had no problems. It had a bit of a recoil issue with supersonic suppressed, but nothing too serious, I'm told.

biffj
08-30-2018, 07:41 PM
I finally got the measurement for the flange on the barrel....its .188-.194" I'd use the thicker one to start with.

I never knew HMG built a blackout or that it worked. I'm sure when that conversion comes up it will get figured out. Reliability is more important than just having it there. In addition, the recoil issue isn't just an annoyence. It is something that will beat the gun up over time and it may be a very short time.

Frank

Xnke
08-31-2018, 08:50 AM
It was also an LC, which had recoil issues to start with. Blackout barrels are available already, as HK93 barrels. Turn the OD to profile and press them in, and you're ready to run with the factory locking piece if you're not running supersonic ammo with the suppressor. Subsonics with and without the suppressor still cycled the action fine, and supersonic without the suppressor worked fine. The suppressor + supersonic ammo caused enough bolt speed to push the gun into slam-firing and beating up the plastic stock.

Sampedro
09-05-2018, 11:47 AM
...

I've been learning a lot about the model L and I think a lot of the problems the guns had were due to improper setup of the bolt and carrier. Bolt gap is really important on these and a little big or a little too small makes a huge difference in bolt velocity. When it gets too high things start getting damaged. From what we've seen ideal is from about .006" to .015" with .010" being about ideal. Sorry Sampedro, I don't speak metric for sizes or temps. I know 1mm is .03937 inches.....
have a look at whatever rifle you're using and see if you can tell how large the bolt gap is.
Frank

Had someone check bolt gap today (rank has some perks) in the sixteen Ls in the company.
14 of them went from .0015 in one of them, to .005 inches (.04 to .13mm)
The other two, one of them marked as very unreliable, had .006 inches (.15mm)

Since the gauges were some cheap chinese stuff (but thankfully marked in inches and milimeters), and there are things I prefer to see for myself, I will check them in a couple weeks when we get back from the next "field trip" to the desert (were several spaghetti westerns were shot in Almeria).

I hope you can get some use of the data.

Xnke
09-05-2018, 04:21 PM
That is very good information to know. I am planning to aim for a 0.003 bolt gap, armed with that information. It seems VERY tight compared to the HK93/91 style rifles, however...it's a different rifle, with different design. If a reliably running gun ranges from 0.0015 to 0.005, that's what it should be.

holescreek
09-05-2018, 06:07 PM
I sure read that post different than you did, I read it as he has 14 rifles that have too little bolt gap. My first thought was the armorers aren't doing their job since a roller change would fix them.


"From what we've seen ideal is from about .006" to .015" with .010" being about ideal."

biffj
09-05-2018, 07:08 PM
I'm kind of thinking the same thing....too little gap. I will agree that the L seems to like a smaller gap than the HK stuff or the C model Cetme but I'm thinking more like .005-.012" is good. In previous tests using a high speed camera we found that with the bolt gap too low as in .003" or smaller and with the bolt gap too large as in .015" we were getting much higher bolt velocity. The sweetheart range seems to be around .006"-.010". Since the barrel has a shoulder to act as the stop rather than just a cross pin the gap won't shorten up as you shoot like the others do either. Even without a pin to hold the barrel in place you can run the thing safely. All the load is pushing the barrel flange against the shoulder in the trunnion up front and the bolt rollers against the trunnion angles in back. The retaining pin is there in the L to keep the front sight base from moving if the barrel rotates in the trunnion. I've yet to see one move though and I've pressed a few in and a few out too. It wasn't easy to remove them......

Frank

Sampedro
09-06-2018, 11:57 AM
Company level armorer doesn't know much about the Ls (he was surprised when I showed him STANAG mags getting in without banging them, and falling freely just pushing the mag release).
And any repair like a roller change had always been at brigade level armorer or higher. Heck, I have never seen rollers in the official spares catalog system, just complete bolt heads.
Nowadays, I doubt there are even any spares outside of swapping parts from other rifles in your unit, IF you are alowed to. On the other hand, the army still has in storage around 15000 brand new CETME L.


As I said, I will check those gaps personally (even if just out of curiosity), but probably could not do it in two weeks, at the least.

Xnke
09-06-2018, 03:01 PM
The work you put in on this is much appreciated, Sampedro. I doubt we could find a better source of information for original CETME-L rifles in the states, there are simply not enough of them assembled and running yet. And due to all the parts kits being bought up just as soon as a usable receiver flat became available, a LOT of people who wanted to build one didn't get the opportunity-You kinda had to buy on faith and hope parts materialized if you wanted one.

Unfortunately I doubt that situation will change, the only current source for 922R parts is HMG, and without kits available to sell their parts, I wonder how long what they have will last.

Xnke
09-16-2018, 10:42 PM
So I placed the order for the HMG flat bending jig friday-and got a call a few hours later asking if I would like to wait for a jig to be built (they are back ordered) Or if I would like to send in my receiver flat, with the denial bar fitted, and have HMG do the laser markings (of my choice, no less) and fold the flat for me.

Well yeah, that sounds pretty good to me, seeing as I can't just pick up another parts kit easily to build more of these in the future.

I'm working on the laser DXF's now to send them, and the flat will be packed and ready to ship in the morning.

I'm thinking on the Magwell:

JTNW
CETME-L
6.8-SPC

Then above the trigger guard where the serial number goes:

LDMR-267717

On the back where the Santa-Barbara stamp was, I have my own very similar gear-and-wrench logo, with JT instead of SB, and a crescent wrench instead of the sword.

I think this will work out fine.

holescreek
09-17-2018, 12:22 AM
Sounds like a win-win! It's not like those flats are easy to come by anyway.

scottz63
09-17-2018, 06:41 AM
That sounds like a great deal!

Xnke
09-17-2018, 09:43 AM
That's what I thought. Now, next on the list are a pair of GHG-1 style rail stiffeners, and extending the barrel trunnion. Extending the trunnion is going to be difficult, because of the way the cocking handle pin is fitted in place. (It's already difficult without the barrel extension) But, I think it is necessary, so it'll have to be figured out.

I've decided to up the barrel diameter from 0.650 to .800 through the extended trunnion, through the handguard, and then step down to 0.650 out past the end of the handguard. I am working on a new front sight assembly or modifying the existing front sight, to allow for both the larger barrel diameter and also the free-floating barrel, which means welding it to the end of the cocking tube. (the cocking tube on these rifles is a SUBSTANTIAL bit stronger than the Cetme-C cocking tube!)

holescreek
09-17-2018, 10:24 AM
Oddly enough I was rebarreling the GHG1 yesterday and had to cut the trunnion extension off to do the job. I had to modify it to get the new barrel in then welded it back on once the barrel was pressed. Luckily I found the CAD drawing and knew exactly where to cut through the original weld.

Xnke
09-17-2018, 11:59 AM
In the Cetme-C, the cocking tube rotates a bit and the cocking handle pin comes out a hole to the side of the barrel. This lets you cut a groove in the extension that would let the cocking handle pin come through, but in the Cetme-L, the pin drops in and and out the top. This would make it seem to be a simple job, removing the pin through the top hole...but it isn't. Extending the trunnion will close access to the bottom side of the cocking tube so I am not sure how that will affect how I install and remove the cocking handle pin yet.

On the rail stiffener T-plates, do they have a radius inside the plate that matches the reciever, or are they square cut? Where the T-profile is, I mean-I expect there to be a radius at the front where the trunnion sits.

holescreek
09-17-2018, 12:54 PM
My T rails look like this:

54659

Xnke
09-17-2018, 03:17 PM
Those look easy enough to replicate. I was looking at turning them from round stock, boring the length, then counterboring the front to fit the trunnion, then splitting them out and milling the rails in the mill.

I *could* do it all in the lathe at home, but it's no big deal to use one of the unloved bridgeports at work.

biffj
09-17-2018, 07:06 PM
I wasn't too keen on the cocking handle pin in the L when I first saw it. With the barrel in place it was a pain to remove. Once I got everything apart though it kind of grew on me. The biggest problem is the ball detent in the top of the pusher. If the grease gets old and stiff it makes life tough. Once cleaned up and oiled its easy to use.

If you're going to make all the new trunnion parts, extensions and so on you might as well make up a new cocking tube as well. Make it so the front end is removable and then you won't need the pin removed at any particular place. Pull the plug out the front and push the assembled handle out. No need for a front sight base, put a rail on the receiver. The iron sights suck really bad. We put a rail on a test rifle and it is so much better I'm shocked. It actually co-witnesses the irons when you use a military aimpoint too so no need for high cheekrests or anything.

I like the idea of the reinforcing rails too. After looking at a bunch of roller lockers that had a lot of rounds downrange I noticed the rails get a bit lumpy where the rollers bounce outward. One spot where the bolt hits the round in the mag, another just behind the trunnion where the rollers try to pop out on recoil, another at the rear where the bolt carrier hits the buffer and bounces the rollers out....I don't think it hurts anything but I like those nice solid looking rails.

Don't plan on using the rollers to adjust bolt gap. The size range is really small and small to large typically gains or loses 1-2 thousandths in gap max.


Frank

holescreek
09-17-2018, 08:17 PM
After looking at a bunch of roller lockers that had a lot of rounds downrange I noticed the rails get a bit lumpy where the rollers bounce outward. One spot where the bolt hits the round in the mag, another just behind the trunnion where the rollers try to pop out on recoil, another at the rear where the bolt carrier hits the buffer and bounces the rollers out....I don't think it hurts anything but I like those nice solid looking rails.

I've only dealt with Cetme C's and have never heard of the rollers denting the rails where the bolt contacts the rear of the cartridge to strip it out of the mag. The dents behind the trunnion is always from the bolt head hitting the rear edge of the trunnion as it enters, because the rails aren't pushed in far enough when the trunnion was welded into the receiver. The dents back at the buffer indicate that either the recoil spring is too worn or that the wrong locking piece is in place.

rebel49
09-18-2018, 12:53 AM
Mike I've seen the roller dents caused by of all things the buffers that sit inside the receiver, you know which ones I mean.

holescreek
09-18-2018, 10:31 AM
Mike I've seen the roller dents caused by of all things the buffers that sit inside the receiver, you know which ones I mean.

Yeah, if the carrier is traveling too fast anything that gets in the way will deploy the rollers with the sudden stop. I've got a couple nice rail dents in the bullpup already. One set from trying the suppressor with the 60 degree locking piece, and a second set from trying out a new idea of replacing the original rubber buffer material with a very thick die spring. My test Cetme also has some nice dents from the suppressor trials.

Xnke
09-18-2018, 06:35 PM
If you're going to make all the new trunnion parts, extensions and so on you might as well make up a new cocking tube as well. Make it so the front end is removable and then you won't need the pin removed at any particular place. Pull the plug out the front and push the assembled handle out. No need for a front sight base, put a rail on the receiver. The iron sights suck really bad. We put a rail on a test rifle and it is so much better I'm shocked. It actually co-witnesses the irons when you use a military aimpoint too so no need for high cheekrests or anything.

Splitting the front of the cocking tube leaves no place to hang the handguard. Unless it's split back where the front edge of the slot is, it wouldn't help anyway, can't take the cocking handle assembly out through the front due to the way the thing is shaped.

I'm thinking just remove the pin and replace it with a shouldered screw, and lock it down with a punchmark.

rebel49
09-19-2018, 12:52 AM
Blackjack buffers has an excellent replacement buffer for the buffer assembly.

Xnke
11-08-2018, 08:31 AM
Working on the molds for the new furniture this week. Might have the pistol grip mold positive ready saturday or sunday, not sure how much time I'll have...

because I bought a new (to me) Logan 820-1 Lathe. It's almost new, the bed shows practically zero wear. It IS five different colors, though, and all sloppily applied except the original Logan Blue-grey.

I'll be stripping and repainting it, and setting up with full coolant capabilities (mine came with the full coolant tray and recirc pump, and coolant-through headstock and tailstock adaptors!)

Unfortunately, even though this is a -1 lathe, I did not get the lever tailstock, just a normal one. I just can't bring myself to pay the 350$ for the one on Ebay right now, so I may have to wait and build a ram type attachment for the tailstock. It will be immensely helpful in cutting the chamber fluting.

holescreek
11-08-2018, 10:12 AM
I had a Logan 820 for several years that came with both tailstocks. I got tired of tripping over the turret tailstock and sold it to a guy in KS.

Xnke
11-08-2018, 04:17 PM
Well, from what I can see, the 820-1 had a lever action tailstock that would work great for drilling and slotting internal keyways, and the 820-2 had the 5 position turret tailstock with the capstan wheel type handle. There was another 820-21 that came with the turret tailstock, and a double-toolpost cross slide that had a lever on it. All would be cool to have but I can totally see how tripping over them would get old. I've seen examples of all three on ebay lately, so eventually I will pick up a lever tailstock, as it combines both the handwheel feed and the lever action in one unit.