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Thread: Pistons and violent early extraction...

  1. #41
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    Quote Originally Posted by MistWolf View Post
    Let me add that until the bullet uncorks the gases in the bore are only traveling as fast as the bullet.

    Robb, I'm pretty certain that the expansion chamber reaches operating pressures after the bullet uncorks. I believe the pressure and density of the residual gas is what the rifle is operating on
    It may.
    But I think we would agree that there has to be enough gas volume fast enough to nearly fill the system with gas before the bullet leaves. If it didn't need to the gas port would be directly behind the barrel threads.
    Once the bullet leaves the barrel most of the gas is going to follow the path of the bullet. Gases are much like electricity they follow the path of least resistance, bigger hole being the barrel not the gas port.

    From what I remember Maj. Muth and LtCol Lutz taught me at Quantico was that a M16A2 needs 20% of the available gas to run properly. I don't know how that figures into DI CAR gas, piston, and DI guns with less barrel between the gas port and muzzle, but common sense tells me they would need more gas.
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  2. #42
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    Quote Originally Posted by MistWolf View Post
    Let me add that until the bullet uncorks the gases in the bore are only traveling as fast as the bullet.

    Robb, I'm pretty certain that the expansion chamber reaches operating pressures after the bullet uncorks. I believe the pressure and density of the residual gas is what the rifle is operating on
    Sure the gases are pushing the bullet so they can't travel faster than the bullet in the bore but they can go down the gas tube faster than they can push the bullet down the bore. As soon as the bullet leaves the bore the pressure in the barrel drops to 0. The dwell time between where the bullet passes the gas port and exits the barrel is when the gas tube and carrier (cylinder)fill with pressure and start moving. The inertia continues to move the carrier to the rear after the bullet has left the bore unless the rifle is over gassed where the carrier moves rearward and extracts the case too early. Cases landing ahead of 3 O'clock, breaking extractors and flames out of the ejection port are all signs of an over-gassed rifle.
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  3. #43
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    The whole short stroke piston idea reminds me too much of smacking a hammer with another hammer. While the (debatable) reliability may be better I dont think the long term durability is as great with a piston ar.
    I think the main advantage of a spring like pws is so you dont have to mess around with making sure the bolt is forward after disassembly to insert back into the upper. The looseness of the ruger sr556 bolt and how it flops around so much makes it a nuisance to align the cam pin back in the track.
    Last edited by VIP3R 237; 03-13-12 at 23:45.
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  4. #44
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    Just keep in mind that when the bullet uncorks, pressure is escaping from the bore through both the muzzle and the gas port. Of course at that point, the majority will be escaping at the muzzle.

    If you watch slow motion video of a bullet exiting a barrel, you will see that the gases vent under pressure for quite some time after the bullet uncorks.

    I do not know how fast the expansion chamber pressurizes once the bullet clears the gas port. If the velocity of the gases drop to subsonic when passing through the port, they won't reach the expansion chamber until after the bullet exits. I'm uncertain how passing through a smaller port affects the supersonic flow of pressurized gases
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  5. #45
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    Well, I am no gunsmith, but I have been doing some reading from several different sources.....in regards to the OP's question.

    It seems that the most important factor in regards to avoiding excess strain on the bolt lugs, is the timing of the unlocking and cycling, and not how the gas pushes forward on the bolt or the impact of the oprod/tappet on the rod face on the carrier.

    The HK416 is cycling faster than a DI, so has a heavier buffer to compensate (150 grams). I am sure this provides sufficient carrier lag, so that the pressure has subsided, and the casing has shrunk before the bolt is unlocked, ensuring proper extraction and ejection.
    Last edited by Arctic1; 03-14-12 at 01:34.

  6. #46
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    Quote Originally Posted by Arctic1 View Post
    ...It seems that the most important factor in regards to avoiding excess strain on the bolt lugs, is the timing of the unlocking and cycling...
    You're correct and, as you pointed out, this fact nearly got lost in the shuffle
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    Quote Originally Posted by Arctic1 View Post
    Well, I am no gunsmith, but I have been doing some reading from several different sources.....in regards to the OP's question.

    It seems that the most important factor in regards to avoiding excess strain on the bolt lugs, is the timing of the unlocking and cycling, and not how the gas pushes forward on the bolt or the impact of the oprod/tappet on the rod face on the carrier.

    The HK416 is cycling faster than a DI, so has a heavier buffer to compensate (150 grams). I am sure this provides sufficient carrier lag, so that the pressure has subsided, and the casing has shrunk before the bolt is unlocked, ensuring proper extraction and ejection.
    This is correct.

  8. #48
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    Quote Originally Posted by MistWolf View Post
    I'm uncertain how passing through a smaller port affects the supersonic flow of pressurized gases
    Ever use an air nozzle with compressed air? How about a pressure washer? Same physics either way

    The smaller the hole, the higher the velocity...up to a point (density and mass come into play)...this is of course relative to the pressure behind the air (gas) to begin with.

    Its actually the same whether its gas or fluid...a pin hole in a hydraulic line with enough pressure on it will spout a stream that will slice your arm off like a samurai sword...the same is true for high pressure gasses, but the range is shorter because the gas has less momentum (slows down in the atmosphere)


    I know...I'm digging up old threads...but I'm reading to learn and thought what I posted here may be of some relevance.

    I'm late the piston party...and playing catch up....and no, I'm not a physics professor...I'm a heavy haul truck driver, but my IQ is a bit higher than room temperature.

    EDITED TO ADD: A smaller gas port would also lower the volume of gas that went through during the firing process...because of the fixed amount of time that the pressure was there....this turned into a deep subject, LOL. I suspect many of you already knew the smaller port lowered the volume...we've all read about over gassing...which is exactly what some are saying the piston system amounts to because of the smaller chamber....if it is smaller...I honestly don't know yet.

    At least give me credit for trying to understand, LOL...and I will understand before its over...and its not about me saying one is better than the other...its like the man said earlier...I just want to understand them both, particularly the piston though because thats what I have.


    Anybody ever compared gas port sizes on any of the new piston guns to a DI? I may be wrong but this seems relevant to how hard the rod hits the carrier.....................I just remembered the adjustable gas block on my Sig 516, it vents the excess gasses out under the handgurads....so I guess that answers the overgassed question.
    Last edited by Ridgerunner665; 08-07-13 at 01:38.

  9. #49
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    Quote Originally Posted by Ridgerunner665 View Post
    Ever use an air nozzle with compressed air? How about a pressure washer? Same physics either way

    The smaller the hole, the higher the velocity...up to a point (density and mass come into play)...this is of course relative to the pressure behind the air (gas) to begin with.

    Its actually the same whether its gas or fluid...a pin hole in a hydraulic line with enough pressure on it will spout a stream that will slice your arm off like a samurai sword...the same is true for high pressure gasses, but the range is shorter because the gas has less momentum (slows down in the atmosphere)


    I know...I'm digging up old threads...but I'm reading to learn and thought what I posted here may be of some relevance.

    I'm late the piston party...and playing catch up....and no, I'm not a physics professor...I'm a heavy haul truck driver, but my IQ is a bit higher than room temperature.

    EDITED TO ADD: A smaller gas port would also lower the volume of gas that went through during the firing process...because of the fixed amount of time that the pressure was there....this turned into a deep subject, LOL. I suspect many of you already knew the smaller port lowered the volume...we've all read about over gassing...which is exactly what some are saying the piston system amounts to because of the smaller chamber....if it is smaller...I honestly don't know yet.

    At least give me credit for trying to understand, LOL...and I will understand before its over...and its not about me saying one is better than the other...its like the man said earlier...I just want to understand them both, particularly the piston though because thats what I have.


    Anybody ever compared gas port sizes on any of the new piston guns to a DI?
    Every op rod AR I have seen cycles faster than a standard AR system.

    The HK is a great example, to counter that they use a 8lb action spring and a 5.3oz buffer which is at the H3 level. Even then it still cycles faster than a Colt 6920 which comes with a H2(4.3 oz buffer) and a 5.5-6lb action spring.
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  10. #50
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    Quote Originally Posted by sinlessorrow View Post
    Every op rod AR I have seen cycles faster than a standard AR system.

    The HK is a great example, to counter that they use a 8lb action spring and a 5.3oz buffer which is at the H3 level. Even then it still cycles faster than a Colt 6920 which comes with a H2(4.3 oz buffer) and a 5.5-6lb action spring.

    Agreed...

    It has to be faster...because the work starts sooner...the piston system doesn't have to wait for the gas to get to the carrier/bolt.

    So now we're back to the unlocking thing...where you were when I got here, LOL. Before I can comment on the unlocking timing I'm gonna have to find out exactly how it works in the DI system...I'm not sure which side of the rings the gas enters the carrier on...nor have I ever given any thought to the actions the bolt goes through during firing.

    Got some studying to do...but its bed time for now...gotta roll a long way tomorrow, delivering to Ft. Stewart...and I'm in Indiana.

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