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Thread: Cetme Locking Pieces for Suppression?

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    holescreek's Avatar
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    Cetme Locking Pieces for Suppression?

    A while back I started experimenting with locking piece angles for shooting suppressed. I used the target Cetme with the 25" barrel because it was already threaded 5/8"-24 for the adapter for my Rugged Surge suppressor. The short story is that the rifle shot fine with and without the suppressor with a LP of 40 degrees but would only work with a 35 degree (reliably) suppressed. Last week I had some spare time and made up some 37 degree LPs to see what the results were. After some though I decided that the test would only be interesting to me because there aren't many long barreled Cetmes around. So a few days ago I made an adapter for the suppressor in M15x1 and put it on my test rifle.

    20180614_121117.jpg

    20180614_122554.jpg

    I took Both Cetmes to the range Saturday to make a video with and without a suppressor using a 50, 45, 40, 37, and 35 degree locking piece. I set up the movie camera (and started it!) and my buddy was operating the high speed camera set at 300 frames per second. After the dust settled I went back to review the movie camera and it was dead. All I left with was the (soundless) high speed video clips. I have a couple of experiences to share though.

    I began with the 45 degree LP because it was already in the 18" rifle. Of course the rifle fired fine as I'd tested it with both the 45 and 40 in it prior, but never with a suppressor. When I added the suppressor the recoil was so violent that it was uncomfortable to shoot and I discovered later on that my shoulder was bruised . I also discovered two peen marks in the rail at the rear of the stroke that I didn't remember being there. And this was just 5 shots suppressed with a 45 degree LP. I didn't bother to try the 50 degree LP because it would be even worse. As the LP shoulder angles decreased the rifle became easier to shoot.

    Before testing I was pretty sure that the 18" rifle was not going to be able to function without the suppressor using both the 35 and 37 degree locking pieces because the 25" wouldn't. I was very wrong. I still can't wrap my head around why, but the rifle functioned flawlessly suppressed and unsuppressed with both LP's and was very pleasant to shoot.

    So I left the range both pissed off (camera) and confused. My confusion is centered around why an 18" barreled Cetme would function, unsuppressed, with a 35 degree LP when my 25" barreled Cetme would not. I had both rifles with me and immediately after shooting the 18" barrel with the 37 I put it in the 25" barreled rifle and it would not cycle unsuppressed. I'm going to be thinking about this for a long time.

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    Senior Veteran Jonathanwild's Avatar
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    How well would a CETME Suppress compared to an AR-10? In sound level and tone? It would be interesting to compare the operating systems to see which is quieter

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    holescreek's Avatar
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    The issue with comparing sounds comes down to metering. Cheap meters don't catch the peak of the sound, so all that's left is how it sounds to you.

    I don't have an AR10 so can't make an educated comparison. I was happy with the suppression at 35 & 37 degrees.

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    Senior Veteran weasel_master's Avatar
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    Pretty interesting observation. Exactly opposite of what I'd expect.
    Czar of all the Michigans
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    holescreek's Avatar
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    I found a video of a G3 running both suppressed and unsuppressed with a 36 degree LP. The only thing affected is the cyclic rate, which changes with the back pressure supplied by each different suppressor tested.



    What isn't making any sense is that the 25" barrel should be producing more back pressure than an 18" barrel, so it should be more likely to operate with the 35 & 37 LP's even more efficiently that the 18". The major difference (aside from length) is that the 18" barrel is an original NATO chambered hammer forged barrel and the 25" is one I chambered and fluted. It may come down to the flutes just not letting as much gas past the case walls.

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    Senior Veteran weasel_master's Avatar
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    Sounds like you've got a reason to make your own 18" barrel and do another build now to test the theory.
    Czar of all the Michigans
    In memory of Johnhttp://act.alz.org/goto/Weasel

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    Senior Veteran sdk1968's Avatar
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    this is an awesome looking gun with the can.

    i just never think of sticking a can on one... but this does look good.

    say what you mean & mean what you say!
    TEC Tactical=SOT/07 i work there.

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    One detail that is often missed in "backpressure" discussions is that there is no more back pressure with a longer barrel than with a shorter one. Pressure drops as the bullet goes down the barrel regardless of the length. Length just dictates how the pressure will drop. Muzzle pressure out of a 20" barrel on an AR 15 in 223 is about 8-10,000 psi. With a 10in barrel its more like 15K and a 7.5in barrel is over 20K psi. This is really important if you're building suppressors and need to know inlet pressure. It also shows how the pressure drops pretty rapidly as the barrel increases in length. Doc Dater from Gemtech did some interesing articles in SAR magazine on pressure changes with length.
    Having said all that, the suppressor on the end of a 20 in barrel will have less effect on the operation of the action than the same suppressor on an 18 in barrel. The suppressor does not increase the back pressure but rather acts like an accumulator and retains what pressure it has longer. This is called dwell time or time under pressure. By increasing the dwell time it retains a higher pressure on the system longer. You're not getting any more pressure but it pushes longer and that can increase bolt velocity which is the part we normally see or feel. Looking at things from that perspective might help understand why the various barrel lengths and suppressors work how they do with the locking pieces.

    I rebuilt some suppressors for a friend and one of them was for an HK 91 with a registered sear pack. The old baffles were quite simple flat plates with raised centers. It was one of the Sionics types with the wierd spiral tube setups as well. This setup increased the rate of fire from a little over 600 to almost 800rpm using the same ammo. I suggested to him that he go from the 45deg locking piece to a 40 and that helped a lot. Once we built new K type baffles the rate of fire increase wasn't as high. Probably his rifle really needed the 40deg locking piece to begin with.
    Frank

  9. #9
    holescreek's Avatar
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    Quote Originally Posted by biffj View Post
    This is called dwell time or time under pressure. By increasing the dwell time it retains a higher pressure on the system longer. You're not getting any more pressure but it pushes longer and that can increase bolt velocity which is the part we normally see or feel. Looking at things from that perspective might help understand why the various barrel lengths and suppressors work how they do with the locking pieces.
    Frank
    Great info and very interesting. I'm going to have to wrap my head around it to figure out how it's different if the end result is that a longer barrel retains the pressure longer (dwell time) and puts more pressure against the bolt head. It was suggested in another forum that the differences between rifles I experienced may just come down to the fluted chambers, the military one allowing more gas to pass back to the cartridge and bolt head. The 25" is a target barrel and performs that function well, so I can just get used to swapping LP's out as needed.

    I pulled the barrel off of the gun I used for this test (pictured above) and reset it to gain some more gap and to right a wrong I made when I first built it. I had reused an original 5mm barrel pin which allowed the barrel so shift forward over time and the gap had closed up to .009" from the original .016". This time I reamed the hole oversized and made a .2037" pin to fit.

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    If your chambers were identical i'd reason to guess that the powder charge has reached it's peak pressure and adding barrel length is not allowing more burn time on the powder since it has peaked. I seriously doubt this to be the case. I would say that 1. unless you used a chamber reamer that was a 7.62 nato reamer with about .300" of throat and free bore and 2. since it is not fluted that same as a milspec barrel i.e. flutes go past the shoulder into the neck and into the throat. I'd say your chambers are no where near close enough to allow an even swap on LP's to run suppressed or unsuppressed.

    As to barrel fluting I don't know why anyone has not looked at EDM and had a few electrode made two roughers and a finisher to do the job. It would take far longer to make the electrodes than EDM the chamber.

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