Page 2 of 13 FirstFirst 123412 ... LastLast
Results 11 to 20 of 129

Thread: Cetme L build planning

  1. #11
    Senior Veteran
    scottz63's Avatar
    Join Date
    Apr 2014
    Location
    Central Mo.
    Posts
    3,083
    Rep Power
    220
    Looking forward to seeing more progress on this.
    14EH AIT Instructor-PATRIOT Fire Control Enhanced Operator/Maintainer

  2. #12
    Veteran
    Join Date
    Nov 2017
    Posts
    57
    Rep Power
    11
    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.

  3. #13
    Veteran
    Join Date
    Dec 2009
    Posts
    91
    Rep Power
    130
    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.

  4. #14
    Senior Veteran
    scottz63's Avatar
    Join Date
    Apr 2014
    Location
    Central Mo.
    Posts
    3,083
    Rep Power
    220
    14EH AIT Instructor-PATRIOT Fire Control Enhanced Operator/Maintainer

  5. #15
    Veteran
    Join Date
    Nov 2017
    Posts
    57
    Rep Power
    11
    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.

  6. #16
    Veteran
    Join Date
    Jan 2016
    Posts
    55
    Rep Power
    35
    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...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
    Last edited by biffj; 07-13-2018 at 11:12 AM.

  7. #17
    Veteran
    Join Date
    Nov 2017
    Posts
    57
    Rep Power
    11
    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.

  8. #18
    Veteran
    Join Date
    Jan 2016
    Posts
    55
    Rep Power
    35
    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

  9. #19
    Veteran
    Join Date
    Nov 2017
    Posts
    57
    Rep Power
    11
    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:




    After opening the bolt face, but prior to polishing:





    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.

  10. #20
    Veteran
    Join Date
    Nov 2017
    Posts
    57
    Rep Power
    11
    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.

Similar Threads

  1. A Big Plus 1 For Planning
    By 7.62guy in forum Seller/Buyer Feedback
    Replies: 3
    Last Post: 05-07-2013, 09:18 PM
  2. Another +1 for Planning
    By pidaster in forum Seller/Buyer Feedback
    Replies: 0
    Last Post: 02-15-2013, 06:56 PM
  3. Build planning for CETME - Modelo G Hypothetical Design Ideas
    By r.erichsen in forum Cetme/HK Building Resources
    Replies: 13
    Last Post: 09-16-2011, 04:28 PM
  4. MSG90A2 build- Cetme Build Tutorial
    By dirty8 in forum Cetme/HK Building Resources
    Replies: 39
    Last Post: 04-10-2011, 09:14 AM
  5. Cetme Build, G3 or CETME Rear Sight?
    By landtoy80 in forum Cetme/HK
    Replies: 9
    Last Post: 12-04-2008, 10:36 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •