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

  1. #11
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    How much stroke the bolt carrier needs to move to unlock the lugs is part of the engineer's concern. Once the lugs are unlocked, the case is driving the bolt carrier group back due to residual gas pressure in the bore.
    It is the moving mass of the bolt carrier that drives the bolt carrier to the rear. Momentum. Newton's First Law.
    My brother saw Deliverance and bought a Bow. I saw Deliverance and bought an AR-15.

  2. #12
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    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.

  3. #13
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    Interesting test.

    Do it.
    We miss you, AC.
    We miss you, ToddG.

  4. #14
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    A few things on this thread so far:

    1- Long stroke versus short stroke definitions are vague at best. I an still waiting on an actual firearm engineer to weigh in on what the working definitions are. More important than how far the piston travels is how much reciprocating mass the operating system requires. Any mass outside the direct bore path will cause the gun to bounce more than one that has all mass within the recoil path. Any mass forward or behind the breech sealing device will cause the gun to bounce more than one with a shorter overall length. Any force exerted on the bolt that is outside the recoil path will cause more bounce when compared to an identical operating system with more centered operative force.

    2- Much of the AKs reliable operation is due to loose fit between operating parts and robust magazines.

    3- Rearward travel of the bolt and bolt carrier group in a DI AR are absolutely linked to gas from the propellant. Gas volume and duration must be properly balanced with the action spring and buffer weight to ensure proper function. Gas volume is determined by the size of the gas port, which is why different length gas systems require different gas port sizes. Short-stroking guns are the proof of this. Short stroking is fixed by opening up the gas port. This is why Bushmasters have large gas ports- to work with cheap, lower powered ammo, and why some Colts and LMTs don't reliably work with said el cheapo ammo.

    Don't let different operating systems confuse your understanding of the DI operating system. Blow-back operation is not the same as Recoil Operation, is not the same as gas-tappet operation, is not the same as DI operation.
    Last edited by Failure2Stop; 05-01-10 at 15:34.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  5. #15
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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.
    TROMIX, I think, did a 'siamese M16 with just that set up. They had reversed AR's (back to back) with the gas system of the first feeding the second and vice-versa.
    ... Have to find a pic....
    Pictures found ...
    Tromix Projects



    Last edited by 4thPointOfContact; 05-01-10 at 18:15. Reason: added pics

  6. #16
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    Yes, saw those pics. The principal is the same as a dual turbo installation on a V engine, it helps balance the gas input/output so the engine banks share a similar load.

    I imagine the crossed up gas inputs help regulate the full auto actions by synchronizing their reciprocal firing. It forces them to take turns.

    That would also introduce the concept that pressurizing the bolt carrier is what propels it enough to cycle the action. Which means I could be completely wrong.

    It would be nice if an actual engineer could chime in with pressure graphs and explain it all.

  7. #17
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    Quote Originally Posted by tirod View Post
    That would also introduce the concept that pressurizing the bolt carrier is what propels it enough to cycle the action. Which means I could be completely wrong.
    I hate to be the one to tell you, but you are completely wrong. See my above post.
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  8. #18
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    Due to a severe derail of the thread, I moved Tirod's last post into a new thread. If anyone wishes to continue the education of DI, please head over to:
    https://www.m4carbine.net/showthread.php?t=53043
    Jack Leuba
    Director, Military and Government Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  9. #19
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    the noticeable difference will be in the felt recoil. The long stoke piston will be softer as the short stroke will be more sharp.

    LWRC 10.5 and 14.5 Piston uppers for reference.

  10. #20
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    Quote Originally Posted by tirod View Post


    Any firearm can have reliability issues - what the AK is known for is durability, a different concept. It means it can take a higher level of mistreatment and still function - not that it has a better ability to feed bad ammo or eat dirt.
    I beg to differ. By design the AK can "ead dirt" in better fashion than the M16.



    Quote Originally Posted by jcote258 View Post
    the noticeable difference will be in the felt recoil. The long stoke piston will be softer as the short stroke will be more sharp.

    LWRC 10.5 and 14.5 Piston uppers for reference.
    So you are saying that LWRC 10.5" uses a short stroke and an LWRC 14.5" uses a long stroke. You are mistaken, LWRC uses only short stroke. The 14.5" uses their mid-length system which amounts to a longer intermediate rod, but it is still a short stroke.

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