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Thread: Army Testing Shows Piston Retrofit kits Are Not Less Accurate, And. . .

  1. #31
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    Quote Originally Posted by Noodles View Post
    . . . wear on the bolt were lower . . .
    All but one M4 (W2) and B2 (a semi-auto only) had cracked bolt lugs by 6000 rounds. (the ones with dashes in the table don't count, they didn't finish the test)

    Which, by the way, is cause for failing an acceptance test . . . In fact:

    ALL the weapons tested would have failed had this been an acceptance test.

    W1 - excessive extreme spread, cracked bolt, excessive malfunctions.
    W2 - excessive extreme spread, (It just barely passed for malfunctions, one more and it would have failed)

    Obviously, none of the piston modified would have passed the number of allowed malfunctions, cracked bolt or cyclic rate.
    Last edited by lysander; 07-26-16 at 16:25.

  2. #32
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    BUT......do the acceptance standards include mostly suppressed firing? That adds quite a bit of back-pressure that wouldn't normally be seen unsuppressed, so wear would be accelerated, no?

    NVM, just re-read the first page and due to suppressor/mount issues they didn't fire all that many rounds suppressed.
    Last edited by ABNAK; 07-26-16 at 19:26.
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  3. #33
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    I have not seen an external piston system retrofit that could address issues with some of the base timing events. There's only a few ways to help the issues in the confined base system dimensions. Using the base dimensions as a confine, a much larger degree of stress and strain reductions can be improved with lesser case volume and/or a larger bore diameter to better time events. Within these confines an impingement .222 system can run well with reduced stresses and strains in a 20" barrel based system. Reduce that barrel length and you start to lose space for improvements. The alternative of a bore diameter increase works as well. Would anyone like to compare the possible ranges of stresses and strains in short SBR's for an AR based platform for a caliber selection?
    Our current system is based on dimensions better suited for for a slightly reduced capacity in a 20" selection. Maybe a better way could be to look towards a base with enough space to better suit our current needs. That would be a change in base receiver dimensions, the extremely compact system may change dimensions for a more preferential one for what we do now.

  4. #34
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    Quote Originally Posted by tom12.7 View Post
    I have not seen an external piston system retrofit that could address issues with some of the base timing events. There's only a few ways to help the issues in the confined base system dimensions. Using the base dimensions as a confine, a much larger degree of stress and strain reductions can be improved with lesser case volume and/or a larger bore diameter to better time events. Within these confines an impingement .222 system can run well with reduced stresses and strains in a 20" barrel based system. Reduce that barrel length and you start to lose space for improvements. The alternative of a bore diameter increase works as well. Would anyone like to compare the possible ranges of stresses and strains in short SBR's for an AR based platform for a caliber selection?
    Our current system is based on dimensions better suited for for a slightly reduced capacity in a 20" selection. Maybe a better way could be to look towards a base with enough space to better suit our current needs. That would be a change in base receiver dimensions, the extremely compact system may change dimensions for a more preferential one for what we do now.
    Tom, love your input but could you please run a synopsis of that in terms those of us without a PhD in engineering can wrap our heads around? Kind of a dumbed-down Cliff-notes version?
    11C2P '83-'87
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  5. #35
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    Quote Originally Posted by ABNAK View Post
    BUT......do the acceptance standards include mostly suppressed firing? That adds quite a bit of back-pressure that wouldn't normally be seen unsuppressed, so wear would be accelerated, no?

    NVM, just re-read the first page and due to suppressor/mount issues they didn't fire all that many rounds suppressed.
    Also, the suppressed test used different weapons than the endurance test.

  6. #36
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    Quote Originally Posted by tom12.7 View Post
    I have not seen an external piston system retrofit that could address issues with some of the base timing events. There's only a few ways to help the issues in the confined base system dimensions. Using the base dimensions as a confine, a much larger degree of stress and strain reductions can be improved with lesser case volume and/or a larger bore diameter to better time events. Within these confines an impingement .222 system can run well with reduced stresses and strains in a 20" barrel based system. Reduce that barrel length and you start to lose space for improvements. The alternative of a bore diameter increase works as well. Would anyone like to compare the possible ranges of stresses and strains in short SBR's for an AR based platform for a caliber selection?
    Our current system is based on dimensions better suited for for a slightly reduced capacity in a 20" selection. Maybe a better way could be to look towards a base with enough space to better suit our current needs. That would be a change in base receiver dimensions, the extremely compact system may change dimensions for a more preferential one for what we do now.
    There is another factor, the retrofit kit has to work with the existing gas port, a gas port sized to pressurize a much larger system. And, in the M4 Carbine the gas system is very close to over-gassed to start with as it need to work in arctic temperatures, where you always have drop in port pressure, it has to work if it is dirty, muddy and firing low spec ammunition The address this by going with a smaller piston diameter but still, the gas port could be smaller.

    As to timing, you can increase the time dwell between piston pressurization and carrier movement by placing a small gap between the rear of the piston and the carrier, a .015" gap can buy you some time. Another way is to go with a stepped piston where the gas acts on a smaller diameter initially then the diameter increases after a point to give full power to operate the system.

    If you were going to design a external piston from a blank sheet of paper that fits in the same envelope as a regular AR, but does not use standard barrels, bolt carriers, buffers and maybe even bolts, it would be a lot easier to make flawlessly reliable, or at least as reliable as the base system. Which is why some companies have done just that.

  7. #37
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    That improvement in accuracy sure is interesting, given the web based declaration of the opposite! Thanks for sharing.

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    Quote Originally Posted by lysander View Post
    There is another factor, the retrofit kit has to work with the existing gas port, a gas port sized to pressurize a much larger system. And, in the M4 Carbine the gas system is very close to over-gassed to start with as it need to work in arctic temperatures, where you always have drop in port pressure, it has to work if it is dirty, muddy and firing low spec ammunition The address this by going with a smaller piston diameter but still, the gas port could be smaller.

    As to timing, you can increase the time dwell between piston pressurization and carrier movement by placing a small gap between the rear of the piston and the carrier, a .015" gap can buy you some time. Another way is to go with a stepped piston where the gas acts on a smaller diameter initially then the diameter increases after a point to give full power to operate the system.

    If you were going to design a external piston from a blank sheet of paper that fits in the same envelope as a regular AR, but does not use standard barrels, bolt carriers, buffers and maybe even bolts, it would be a lot easier to make flawlessly reliable, or at least as reliable as the base system. Which is why some companies have done just that.
    Don't forget about the issue concerning the net bolt tension to the BE when comparing the DI to the external piston. The DI bolt has a force applied towards the muzzle, the external piston system does not during unlocking. Looking at the timing of events, we would rather see things that do not add stresses and strains. Comparing a DI system to an external piston is not direct, more time is needed in the external piston for a similar net bolt tension. The base system receiver simply doesn't have enough room to do this as well as a DI. This and the base receiver was never intended to handle the off axis loading of an external piston. With the associated issues, you can't really make the external piston system run as well with the base receiver overall. Too many negatives come up.

  9. #39
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    As configured, all but one of the piston conversion kits resulted in lighter weapons that the standard M4. This is because the standard M4 used in the test were fitted with the Adapter Rail System (ARS), and none of the supplied piston kits allowed installation of the ARS, so manufacturer supplied handguards were used.
    I'm willing to bet the handguards were the primary cause for the changes in accuracy. I can't know for sure, since I don't know which ones were supplied with which pistons, but this could definitely be a big factor.

  10. #40
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    Quote Originally Posted by tom12.7 View Post
    Don't forget about the issue concerning the net bolt tension to the BE when comparing the DI to the external piston. The DI bolt has a force applied towards the muzzle, the external piston system does not during unlocking. Looking at the timing of events, we would rather see things that do not add stresses and strains. Comparing a DI system to an external piston is not direct, more time is needed in the external piston for a similar net bolt tension. The base system receiver simply doesn't have enough room to do this as well as a DI. This and the base receiver was never intended to handle the off axis loading of an external piston. With the associated issues, you can't really make the external piston system run as well with the base receiver overall. Too many negatives come up.
    That brings up an interesting subject, into which we will get more in depth with at a later date.

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