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Thread: I still don't understand the CETME unlocking sequence

  1. #1
    Veteran W.E.G.'s Avatar
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    I still don't understand the CETME unlocking sequence

    I recall reading that when the gun fires, the carrier begins moving back prior to any movement of the bolt.

    Can somebody explain to me what causes the carrier to start moving prior to the bolt moving?

    Is the recoil force more greatly concentrated against the locking piece initially?

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    Quote Originally Posted by W.E.G. View Post
    I recall reading that when the gun fires, the carrier begins moving back prior to any movement of the bolt.




    this is not entirely true

    as the pressure of the cartridge case pushes back on the bolt head, the movement of the bolt head is resisted by the rollers. the rollers are cammed in by the diminishing slope of the locking recesses in the trunion. as the rollers are cammed in, they push back on the locking piece. the locking piece pushes back on the bolt carrier, and as the locking piece is pushed back far enough to allow the rollers to recess inside the bolt head, then the bolt head is freed from the trunion and allowed to reciprocate.

    the bolt head is actually moving as soon as the round is fired, but only a very small amount. through this locking system, a small movement in the bolt head is transferred/multiplied into a large movement in the bolt carrier through a mechanical disadvantage

    remember this is a blowback operated weapon, not recoil operated, and the bolt head is never fully locked into battery, only delayed


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    here's another good pic. as you can see the black outline is the bolt system in battery, the red outline is the bolt assembly as it starts to move. the bolt head does move slightly, but as the bolt head moves the locking piece/bolt carrier move in increasing amounts


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    Veteran texlurch's Avatar
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    Also note how #3 roller moves along the ramped portion. The deeper in the trunnion, the longer the delay, closer to the corner, less delay.

    The deeper the roller in the bolt, the farther back your LP is, which equates to bolt gap. The "sweet" spot along the ramp is in the .004-.020 range... the bigger gap means the roller unlocks sooner, and that is where you get to that nice push, rather than a kick.

    As soon as you pull the trigger, everything starts moving; at the same time the pressure is already bleeding off, as the bullet is gone. So if you have too much delay, the bolt doesn't unlock correctly, leading to short stroking, ejection and feed problems and a harder kick. If it is too far in, it will turn into a bolt action and not even unlock.
    Last edited by texlurch; 03-09-2008 at 04:47 PM.
    There is always room for another AK or 1911.

    As with anything car or gun related, your results may vary!

    "There has been a disturbing increase in things I know nothing about."

    Mr. Obama: I'll keep my freedom, hard earned cash, and my guns.... you Sir, can keep the Change!
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    Veteran W.E.G.'s Avatar
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    Would it be correct to say that the trunion has a less-steep slope than the locking piece?

    If that is the case, is the difference in the slope the factor that allows the relatively small rearward movement of the bolt head to generate the greater rearward movement of the locking piece over a single time interval?

  6. #6
    Veteran texlurch's Avatar
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    Quote Originally Posted by W.E.G. View Post
    Would it be correct to say that the trunion has a less-steep slope than the locking piece?

    If that is the case, is the difference in the slope the factor that allows the relatively small rearward movement of the bolt head to generate the greater rearward movement of the locking piece over a single time interval?
    Just the opposite, if you refer to the above pic.

    The LP angle is really shallow, and the trunnion cut out is more like 45-50 degrees on the roller slope.

    Basically the ratio in the CETME is 4 to 1. For every .001 the bolt head moves, the angles in the LP and trunnion multiply to move the LP ( and hence the carrier, since the LP is attached to the carrier) .004.

    So, on recoil, as soon as the bolt face moves back .001, the carrier moves .004. This rate continues until the rollers come in far enough to pop the LP back and unlock. I think this is where people get confused and think the carrier moves before the bolt. The bolt initiates the movement, but the carrier is moving 4 times as fast, so relative accelleration is faster.

    If you think about what is happening, it explains why the recoil is so smooth when they are set up right. Immediately when the bolt face sees pressure, the bolt is already headed back; in a smooth, controlled manner. And since the heavy bolt carrier is being accellerated by a multiple of 4, it eats up a lot of the bolts inertia. Remember, this is a blow back weapon, the bolt just has this cool but complicated roller system controlling the speed.

    Just as fast, the bullet is already gone and chamber pressure is dropping fast. So now you have just the mass of the bolt and carrier coming back, already slowing down.

    This is also why bolt gap is such a critical measurement, because it relates to the timing of the bolt unlocking, cylinder pressure, recoil, extraction, on and on and on.

    I can pretty much shoot my G3 (.015 gap) one handed and it is like an AK in recoil, maybe less....
    Last edited by texlurch; 03-09-2008 at 08:40 PM.
    There is always room for another AK or 1911.

    As with anything car or gun related, your results may vary!

    "There has been a disturbing increase in things I know nothing about."

    Mr. Obama: I'll keep my freedom, hard earned cash, and my guns.... you Sir, can keep the Change!
    COG# 182, USMC, ret., Upstanding member of WECSOG كافر

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    Veteran W.E.G.'s Avatar
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    OK.

    That makes sense.

    So, the key is, the different angles of the surfaces engaged by the rollers allows the moving pieces to move at significantly different velocities, although the pieces all start moving at the same time.

    Do I have a clue now?

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    Veteran MID's Avatar
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    yup

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    how and whys of stg-45 system

    Also how its history started is the key ,the hows and whys it works at the end of ww2 Mauser built a ''bullet hose'' stg-45 that works with mass production ,low costs and yes -the evil wording and forgotten reason,throw -a-way weapon system ,seeing Germany was about to be over run by Russia and others .Years later , Franco of Spain seen this of Russia maybe over run likes of all of Europe .So cetme was formed ,the rest is history and in the math , how it works ....

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    Senior Veteran SteelCore's Avatar
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    I can pretty much shoot my G3 (.015 gap) one handed and it is like an AK in recoil,

    Yeah, I got the gap up to .017 on my CETME, and it is like an AK... when the gap was .010 or .011, it was kicky, and took about a sec to reacquire the target. At .017, there's none of that. It is mighty scary now to thing that a 762 battle rifle can handle like an 'assault' (medium power cartrige) rifle.

    I read that the delayed roller locking system keeps the weight down on the bolt. If it was direct blowback, the bolt would have to weigh 37 pounds! That means that roller locking system is really doing a lot of work by directing a lot of straight-line force out laterally.

    Also, since the system has no falling breech block or rotating locking bolt parts, the rifle doesn't really twist, kick or torque, and with the buutt lined up directly with the bbl, there's no real muzzle rise (I point this out because proponents of Stoner's AR series act like he totally invented the idea of putting the stock linear with the bolt...which is )

    Sorry, proud CETME owner and fan here.

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