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Thread: Long stroke VS. short stroke gas piston system?

  1. #31
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    .

    The way it was explained to me:

    On a Long Stroke, the piston is connected solidly to Bolt Carrier and moves the same length that the Bolt Carrier moves while it cycles the cartridge. The Long Stroke is a softer, less violent action because the BC is powered its entire length by combustion gas.

    Basically, on a Short Stroke, the piston just touches up to the Bolt Carrier-no positive link, just bumps up against it. Once the gun is fired, the piston will get the BC moving rapidly rearward but the piston will stop at 3/4" or so. Once the piston stops, it isn't pushing the BC anymore but the BC continues rearward under it own momentum to cycle the cartridge. The piston just "catapults" the BC back for the rest of its cycle. This is a more violent action because the piston has to be powered aggressively to insure the BC will cycle when the gun gets dirty, for different ammo strengths, etc.

    Is this clear as mud? Is it correct?

    .

  2. #32
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    Quote Originally Posted by Thomas M-4 View Post
    I have seen this train of thought before what long stroke rifle has a long impulse? From what I can tell the ZB26,Bren,BAR,PKM & AK all vent the gas early IE short impulse. I am sure some where in the firearms world there has been a long impulse design but the above represents to the world what long stroke operating system are.
    No, I'm not saying long stroke= long impulse.

    To me, long or short stroke is how far the piston moves.

    The impluse is what matters more.

    Take the scar with a consistent placement of the gas block and piston.

    The 10" barrel vs the 18" barrel.

    Big difference in duration, but the total impulse and carrier velocity is roughly the same.
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  3. #33
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    Quote Originally Posted by KevinB View Post
    Colt made a Long Stoke piston (Colt's initial M16A2 concept gun of the late 60's or early 70's). We have a few here, the piston is part of the bolt carrier (well it links into it).
    Which appears to have been the basis for the Korean K-2 rifle. The K-2 is a piston AR done right IMO.
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  4. #34
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    Quote Originally Posted by ucrt View Post
    .


    Is this clear as mud? Is it correct?

    .
    I got it explained other way round.

    Long stroke has piston travel all way with bolt back and forth, increasing moving mass and making rifle less stable. Example given - AK

    Short stoke on other hand have piston and rod move only part way with bolt and then move forward at same time the bolt still travels backward. In this case overall momentum of moving masses is reduced (momentum of piston/rod moving forward is deducted from momentum of bolt/bolt carries moving forward as it has opposite vector of force direction). This makes rifle operate less violent and more stable. Matter of timing and moving masses. Example given - SVD

    In AR world both short stroke and long stroke system add considerably less mass so it's may have less impact here. But I never found short stroke system like in G36 or SL8 to be violent. In fact I was always amazed how soft shooting they are.
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  5. #35
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    Quote Originally Posted by tirod View Post
    I thought that at one time. What moves the bolt carrier on a .22 blow back, or the slide on a semi auto pistol? Gas pressure in the case pushing back against the bolt face.

    Roller locked bolts on HK's? Case forcing the bolt back. There's no operating rod.

    I believe DI just opens the bolt. Case pressure pushes the bolt back, rotating the cam pin and forcing the head against the upper track, where the TM states to "lube generously."

    If I had two AR's, I could demonstrate by connecting the gas tube of one to the other, observing whether it would cycle the bolt on the non fired gun, extracting, cocking, ejecting, and reloading. That would eliminate the affect of gas in the barrel.

    I don't even think it would eject.
    No, the gas pressure on the case does not move the bolt back, nor (as some assert) does the gas pressure inside the carrier push the bolt forward to aid in unlocking the bolt. As soon as the bullet clears the gas port, the gas pressure from case web all the way into the carrier is equalized. You need a pressure differential to push something. At this point, the pressure can only move the carrier rearward and the bullet forward

  6. #36
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    Quote Originally Posted by MistWolf View Post
    No, the gas pressure on the case does not move the bolt back
    Ever seen carrier velocity graphs of suppressed versus unsuppressed or watched them under high speed video?
    Cause if you have you would know your answer is incorrect.

    nor (as some assert) does the gas pressure inside the carrier push the bolt forward to aid in unlocking the bolt. As soon as the bullet clears the gas port, the gas pressure from case web all the way into the carrier is equalized. You need a pressure differential to push something. At this point, the pressure can only move the carrier rearward and the bullet forward
    Dude your understanding of the whole thing is just skewed...
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  7. #37
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    In the AR, Case pressure does push the bolt back, about .002" until the locking lugs of the barrel extension stop the bolt, just like with a bolt action.

    Regarding the bolt moving forward, You don't need a pressure difference to push something, you need a force difference.

    In the case of the AR, the equal pressure in the chamber and piston produce different forces because of different areas/ diameters.

    The case has approximately .330" inside diameter.

    The piston on the bolt has approximately .430" effective diameter.

    This force difference does push the bolt forward and away from the extension locking lugs.

    The reaction force pushes the carrier rearward.

    By the time the carrier has moved far enough to start unlocking the bolt, the bullet has left the barrel and chamber pressure drops quickly.
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  8. #38
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    Clint, I see that the thrust to the bolt face will take up any slack there may in the bolt lock up when the weapon is fired.

    However the point I was making is, when the gas system is pressurized and the bolt carrier begins it's motion, there is equal pressure in the case and in the carrier. As such, the pressure would not be pressing the bolt back, it is pushing the carrier.

    Clint, you are correct when I spoke of the pressure not pushing the bolt forward, I did not take the difference in surface area into account. However, the brass case (note I say brass not steel) has also expanded under pressure to fill and seal the chamber, including the headspace and also grips the chamber walls, resisting any movement of the case, forward or aft. I do not know if this stiction is enough to overcome the force difference due to the diameter differences between the front of the bolt and the rear, but I do know that it's enough to significantly reduce bolt thrust. P.O. Ackley wrote about this.

    KevinB, if my understanding of the whole thing is skewed, perhaps you can explain how it really works.

    Self loading weapons are designed to ensure the bullet has exited the muzzle and pressures have dropped to safe levels before the bolt unlocks. This is to ensure that the bolt isn't trying to extract the fired round while it's sticking to the chamber and to keep high speed, high pressure gases from damaging the weapon and shooter. By the time the bolt of the AR unlocks the gases have vented and pressures dropped dramatically. Still, they will be relatively equal on both sides of the piston and the jetting action of the ejecta mostly dissipated. (The ejecta does add significantly to recoil).

    Suppressors change the equation, in particular by increasing dwell time and slowing the escape of gases at the muzzle.

    I do have a couple of questions as I realize every design has it's quirks-
    -What source references to the bolt being pressed forward to aid in unlocking? Is this a theory or has it been measured and proven?
    -What source references there is enough gas pressure on the face of the bolt, after it's been unlocked, to aid in it's rearward motion?

  9. #39
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    Quote Originally Posted by MistWolf View Post
    Clint, I see that the thrust to the bolt face will take up any slack there may in the bolt lock up when the weapon is fired.

    However the point I was making is, when the gas system is pressurized and the bolt carrier begins it's motion, there is equal pressure in the case and in the carrier. As such, the pressure would not be pressing the bolt back, it is pushing the carrier.

    Clint, you are correct when I spoke of the pressure not pushing the bolt forward, I did not take the difference in surface area into account. However, the brass case (note I say brass not steel) has also expanded under pressure to fill and seal the chamber, including the headspace and also grips the chamber walls, resisting any movement of the case, forward or aft. I do not know if this stiction is enough to overcome the force difference due to the diameter differences between the front of the bolt and the rear, but I do know that it's enough to significantly reduce bolt thrust. P.O. Ackley wrote about this.

    KevinB, if my understanding of the whole thing is skewed, perhaps you can explain how it really works.

    Self loading weapons are designed to ensure the bullet has exited the muzzle and pressures have dropped to safe levels before the bolt unlocks. This is to ensure that the bolt isn't trying to extract the fired round while it's sticking to the chamber and to keep high speed, high pressure gases from damaging the weapon and shooter. By the time the bolt of the AR unlocks the gases have vented and pressures dropped dramatically. Still, they will be relatively equal on both sides of the piston and the jetting action of the ejecta mostly dissipated. (The ejecta does add significantly to recoil).

    Suppressors change the equation, in particular by increasing dwell time and slowing the escape of gases at the muzzle.

    I do have a couple of questions as I realize every design has it's quirks-
    -What source references to the bolt being pressed forward to aid in unlocking? Is this a theory or has it been measured and proven?
    -What source references there is enough gas pressure on the face of the bolt, after it's been unlocked, to aid in it's rearward motion?
    the bolt does not move forward, there is insufficient pressure in the carrier to do this, further the ar design relies on residual chamber pressure to ensure extraction. there is an ndia briefing which demonstrates this here: http://www.dtic.mil/ndia/2003smallarms/din.ppt#1

    obviously, this happens over a very small period of time but chamber pressures throughout are always greater than carrier pressures.

  10. #40
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    in modern terms , the op rod is attached to the bolt carrier in a long stroke piston design; in a short stroke design the op rod is not.

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