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Thread: Cetme L build planning

  1. #1
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    Cetme L build planning

    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:



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



    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?

  2. #2
    Senior Veteran pryotex's Avatar
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    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.

    TEC Tactical is a Licensed 07/02

  3. #3
    Senior Veteran Mike928's Avatar
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    Quote Originally Posted by Xnke View Post
    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:



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



    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?
    WBZ

  4. #4
    holescreek's Avatar
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    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.

  5. #5
    Senior Veteran Blackwing's Avatar
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    Quote Originally Posted by Mike928 View Post
    Do you mind sharing some pictures of your "failed" attempt at chamber fluting?
    +1

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    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.

  7. #7
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    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!

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    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

  9. #9
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    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.

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