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Discussion Starter · #1 ·
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/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.
 

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Great explanation, but sometimes one does not want to know lol.
Getting back to details there is still a difference. In your terminology the AR15 expansion is all inside of the bolt so it is a piston inside the bolt system. Basically it is a defecate where it eats system that is the most important take home system
 

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Discussion Starter · #5 ·
Great explanation, but sometimes one does not want to know lol.
Getting back to details there is still a difference. In your terminology the AR15 expansion is all inside of the bolt so it is a piston inside the bolt system. Basically it is a defecate where it eats system that is the most important take home system
And so do blowbacks . . .
 

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And so do blowbacks . . .
Not exactly in the same way. In the AR15 the gas is directed inside of the bolt. In the straight blow back it is directed to inside the receiver in general. The most common blow backs are in .22 LR rimfire that use lubricated bullets with the exception of the few that use a copper jacket. The .22 is a dirty round relative to small amounts of powder and size of the bullet.
 

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I just know that when I clean one (AR15) they are dirtier than my M1, M1A, SIG 556R, etc.

Dirtier, harder to get as clean (combination of the gas/carbon location and the bolt/bolt extension lugs) and cheaper to shoot.

If I shoot it, I clean it. Ten shots or one hundred shots, or two hundred shots. It gets cleaned and inspected and lubed and wipe down before it gets put away (safe or gun case) so it's ready for the next time. More time spent taking care of it and making sure it's clean/lubed/ready means less worry about whether or not it will work when I need it.
 
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In Eugene Stoner's own words (taken from his 1956 patent, US603913A):

...
It is another object of this invention to utilize the energy of the expanding gas developed by the firing of the weapon, for actuating the automatic rifle mechanism directly by use of a metered amount of the gas coming from the barrel. This invention is a true expanding gas system instead of the conventional impinging gas system. By utilization of a metered amount of gas from the barrel, the automatic rifle mechanism is less sensitive to different firing pressures caused by variations in the propelling charge. It is therefore still another object of this invention, to provide a rifle mechanism which is not affected by variations in the propelling charge.
....

Patent US603913A
 

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From the first post:

“The M1 Garand and AK series are also classed as impingement.”

help me understand why the M1 Garand would’ve classed as impingement. It’s gas cylinder looks exactly like the diagram for expansion. Was that supposed to be M1 carbine?
 
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