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Thread: Everything you know is wrong: Impingement vs Gas Expansion

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    Everything you know is wrong: Impingement vs Gas Expansion

    It’s not what you think, in fact, it’s the exact opposite of what you think.

    Impinge, a verb meaning “to strike or hit, especially with a sharp collision.” In gun design textbooks, an “impingement gas system” is one where gas, at high velocity, is directed at a perpendicular surface, and through the momentum of the gas, motion is imparted on the surface.

    Classic examples of impingement gas systems are the Ljungman and the MAS-49.

    Ljungman


    MAS-49


    However, looking at the gas tubes, they actually are inserted into small cylinders machined into the front face of the bolt carrier, and in the case of the MAS, it’s not that small, almost an inch deep. In reality, since the gas is trapped inside a cylinder, some motion is gained through expansion of the gas. In fact, most of it, as pure gas impingement is terribly inefficient. So, what other gas system designs have a small tube inserted into a cylinder?

    AR-18


    Yes, the AR-18 series. Functionally, the AR-18 is the same as a MAS-49, schematically shown below, the only difference is the distance between points A and B. The M1 Garand and AK series are also classed as impingement.

    AR-18/MAS Schematic


    Contrasting the impingement gas system is the “gas expansion system.” In the gas expansion system, high pressure gas is introduced into a chamber, at relatively low velocity, or at a right angle to the direction of motion, or both, and only through expansion of the gas volume is motion achieved. The White gas cut-off system, used in the T25 and M14, is the usual go to example of an expansion system. A rather basic schematic is shown below:

    Expansion schematic


    But, that schematic look awfully familiar, where have we seen that before? That’s right, US Patent No. 2,951,424 – “GAS OPERATED BOLT AND CARRIER SYSTEM”, by one E.M. Stoner. In fact, Stoner refers to this system as a gas expansion system, in the patent, the words “expand” and variations thereof are stated 18 times, “direct” or “impingement”, zero.



    So, the gas tube design of the AR-15 is an expansion piston, and the piston design of the AR-18 is an impingement piston. Since, almost all of the “piston” ARs are based on the AR-18 or M1 style pistons and cylinders, they are impingement systems, and the term “direct impingement” is better suited to describe most the AR “piston” designs than the original gas tube design.
    Last edited by lysander; 11-28-22 at 10:27.

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    Interesting. I never thought to look at the meaning of "impingement" .
    "What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v

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    Good explanation.

    I think of the terms similar to silencer vs supressor. The terms may not be applied according to the technical definitions of each word, but a general industry-wide understanding of:
    DI = force imparted in the action
    Piston = force imparted away from the action

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    If anyone actually made a "direct impingement" system, it would look something like this, and be horribly inefficient.


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    Quote Originally Posted by MegademiC View Post
    Good explanation.

    I think of the terms similar to silencer vs supressor. The terms may not be applied according to the technical definitions of each word, but a general industry-wide understanding of:
    DI = force imparted in the action
    Piston = force imparted away from the action
    Yes, the terms are ingrained into the minds of the AR-crowd. Similar to the term "dwell time," which technically is the time from the ignition of the primer to the unlocking of the bolt, and is important as it controls the chamber pressure at the time of extraction. But the wrong definition is in the minds of the AR community and will never change.

    The reason these definitions are misused by the AR community is because the AR is the first time the the average joe, non-engineer or gun specialist got to play around with the configuration and many of the design parameters of a semi-automatic rifle design. Having heard terms like "dwell", "direct impingement", etc, but not knowing the actual definition, applied they where they thought fit and they became ingrained before the technical geeks could correct them.

    Think about it, before the "AR Spring" in the early 1990-2000s when could the average non-gunsmith play with, not only the barrel length, but the location of the gas port from either end of the barrel, weight of the reciprocating mass and all the little technical details that normally get sorted out before a rifle hits the market?
    Last edited by lysander; 11-28-22 at 10:23.

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    The "AR Spring" brought a lot of pain and stupidity. Aftermarket junk makers cut so many corners, used bad port specs, etc. It brought about the idiot expression about the AR... "It shits where it eats".

    With a lot of shooters exposed to BAD ARs, there was a period of time where the reliability of the design was in doubt. And industry dullards came up with extractor Orings and the piston AR. Combine this with the Advanced Armament suppressor availability, and the popularity of running ARs suppressed? It was one big goat screw.
    "What would a $2,000 Geissele Super Duty do that a $500 PSA door buster on Black Friday couldn't do?" - Stopsign32v

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    Quote Originally Posted by markm View Post
    The "AR Spring" brought a lot of pain and stupidity. Aftermarket junk makers cut so many corners, used bad port specs, etc. It brought about the idiot expression about the AR... "It shits where it eats".

    With a lot of shooters exposed to BAD ARs, there was a period of time where the reliability of the design was in doubt. And industry dullards came up with extractor Orings and the piston AR. Combine this with the Advanced Armament suppressor availability, and the popularity of running ARs suppressed? It was one big goat screw.
    It always cracks me up when the idiots build an AR from cheapest, shittiest parts they can find (cuz its just like Legos) then whine about how AR's are unreliable crap.

    The O'rings were intended as a stopgap not a solution.
    Gettin' down innagrass.
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    Quote Originally Posted by markm View Post
    The "AR Spring" brought a lot of pain and stupidity. Aftermarket junk makers cut so many corners, used bad port specs, etc. It brought about the idiot expression about the AR... "It shits where it eats".

    With a lot of shooters exposed to BAD ARs, there was a period of time where the reliability of the design was in doubt. And industry dullards came up with extractor Orings and the piston AR. Combine this with the Advanced Armament suppressor availability, and the popularity of running ARs suppressed? It was one big goat screw.
    Crane added O-rings because our Colt guns with KAC NT4s were choking. But yeah, I agree with much of what you’re saying.
    RLTW

    Former Action Guy
    Disclosure: I am affiliated PRN with a tactical training center, but I speak only for myself. I have no idea what we sell, other than CLP and training. I receive no income from sale of hard goods.

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    Quote Originally Posted by 1168 View Post
    Crane added O-rings because our Colt guns with KAC NT4s were choking. But yeah, I agree with much of what you’re saying.
    That is what brought about the copper colored extractor spring.

    Of course, if you are not running a suppressor in full-auto, the need for the o-ring and/or the copper spring not there, and the old 0.028" wire spring will work fine.

    But, try and tell people that.

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    Quote Originally Posted by lysander View Post
    It’s not what you think, in fact, it’s the exact opposite of what you think.

    Impinge, a verb meaning “to strike or hit, especially with a sharp collision.” In gun design textbooks, an “impingement gas system” is one where gas, at high velocity, is directed at a perpendicular surface, and through the momentum of the gas, motion is imparted on the surface.

    Classic examples of impingement gas systems are the Ljungman and the MAS-49.

    Ljungman


    MAS-49


    However, looking at the gas tubes, they actually are inserted into small cylinders machined into the front face of the bolt carrier, and in the case of the MAS, it’s not that small, almost an inch deep. In reality, since the gas is trapped inside a cylinder, some motion is gained through expansion of the gas. In fact, most of it, as pure gas impingement is terribly inefficient. So, what other gas system designs have a small tube inserted into a cylinder?

    AR-18


    Yes, the AR-18 series. Functionally, the AR-18 is the same as a MAS-49, schematically shown below, the only difference is the distance between points A and B. The M1 Garand and AK series are also classed as impingement.

    AR-18/MAS Schematic


    Contrasting the impingement gas system is the “gas expansion system.” In the gas expansion system, high pressure gas is introduced into a chamber, at relatively low velocity, or at a right angle to the direction of motion, or both, and only through expansion of the gas volume is motion achieved. The White gas cut-off system, used in the T25 and M14, is the usual go to example of an expansion system. A rather basic schematic is shown below:

    Expansion schematic


    But, that schematic look awfully familiar, where have we seen that before? That’s right, US Patent No. 2,951,424 – “GAS OPERATED BOLT AND CARRIER SYSTEM”, by one E.M. Stoner. In fact, Stoner refers to this system as a gas expansion system, in the patent, the words “expand” and variations thereof are stated 18 times, “direct” or “impingement”, zero.



    So, the gas tube design of the AR-15 is an expansion piston, and the piston design of the AR-18 is an impingement piston. Since, almost all of the “piston” ARs are based on the AR-18 or M1 style pistons and cylinders, they are impingement systems, and the term “direct impingement” is better suited to describe most the AR “piston” designs than the original gas tube design.
    I always thought Stoner's original design for the AR10 (which is referenced above) would have been superior to the later "gas key on top of the carrier". It would be interesting if it would have been retained.

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