Everything you know is wrong: Impingement vs Gas Expansion

[lysander]

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.