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Thread: Gas operated Cetme in .243 Win with an unfluted chamber

  1. #31

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    That rifle looks awesome. You are sure going to get some double takes when you take it to the range. "Ah, nice CETME... Wait, what IS that?"

    Be interesting to see what you learn about the comps.
    Time for the book "The Decline and Fall of the United States of America?"

  2. #32
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    Hi all.
    Sorry for the topic post now. And sorry for my English.

    There is one thing that concerns me concerning the "safety race" (safety distance when opening). I see that the locking piece has been modified to further delay the opening of the rollers. But this run (distance) is still very small If we compare to other gas borrowing system.

    The recoil of the cylinder head holder and the cylinder head have the same distance with the modified locking part or not. The rollers are actually slightly delayed with the new part, but not the safety stroke. Is it sufficient? (Hope you understand what I mean)


  3. #33
    Senior Veteran SgtHorvak's Avatar
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    Ever since I got A LOT of advice from you 2+ years ago on my first CETME build, I have referred many people to your builds and tutorials...THIS rifle is a great example of why!!!

  4. #34
    holescreek's Avatar
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    The locking piece for this build was made at 10 degrees vs. The standard 50 degrees on an original LP. This keeps the rollers locked outward until the bolt carrier is struck by the op rod and due to the shallower angle the carrier must move rearward further to unlock. Also the adjustable gas block regulates the pressure on the op rod piston.
    It has been too long to remember how far the carrier moves before unlocking, I'll try to remember to check my old cad drawing.

  5. #35
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    Thank you for that answer. I think I understood the system. But is this stroke enough not to cause the box to explode on the outside? In the drawing (original vs modified), only step (2) is different but the stroke before full unwinding remains the same.
    When the gaz hit the piston, are there not gases to be released into the chamber yet?
    I mean in simpler terms, isn't unlocking too fast?
    In any case, well done for your work.
    Last edited by Acarian; 10-05-2020 at 05:37 PM.

  6. #36
    holescreek's Avatar
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    When the carrier is in cycle several things are taking place at once. In your first set of drawings (very nice depiction!) you are showing the movement of the locking lever on the bolt head. The longitudinal distance the lever moves (bolt gap wise) is dictated by the inward movement of the rollers starting from locked position until the rollers clear the sides of the trunnion. In theory this distance will be the same no matter what angle of locking piece you use. The bolt can't open until the rollers can fit between the sides.

    I attached a pic of a random 37 degree Cetme LP to show the bolt gap locked (on the right side) vs. the bolt gap when fully unlocked (left side). The total amount of gap movement and the speed at which it occurs is governed by the angle of the locking piece. The smaller the LP angle is, the longer it takes to unlock.

    LP37 pic.jpg

    On this build using the SVT style op rod and 10 degree LP the gas pressure on the bolt face is not strong enough to overcome the 10 degree LP by itself as normally happens in a Cetme. In the Youtube video you can see bolt start to move (in the slow motion of the first shot) because the gas wasn't completely shut off. If the op rod had not moved the bolt could not have opened at all. But even after opening about 1/2" the gas pressure remaining inside the barrel still wasn't strong enough to cycle the action any further.

    I wish I'd have chosen a different caliber but I didn't want to have to deal with magazine issues at the time.

  7. #37
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    OK I understand. Thanks a lot for the clarifications.

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