Now, based on the four previous articles on gun operating mechanisms, some of my readers may be thinking “jeeze, Nathaniel, we already know all this stuff! Why are you telling us this?” Don’t worry! I haven’t forgotten about you, and that’s why I am sprinkling in more advanced topics as we talk about the more basic ones. We just spoke about the principle of gas operation, so let’s jump up a grade to 201 level, and talk about two of the most conceptually obvious incarnations of gas operation. I’m writing, of course, about the Bang, and gas-trap methods of tapping propellant gases.

• Bang-type gas operation: Where gasses are tapped against a forward-moving muzzle cap, which is connected via a seesaw linkage to an operating piston.
• Gas trap operation: Where gasses are tapped into a stationary muzzle cap, and then bled to the rear to act against a gas piston.

Because of their similarities, these two systems of operation are often confused; both use muzzle caps that harness the gas pressure without needing ports drilled in the weapon’s barrel, but they differ in how that gas is utilized. The Bang system is an earlier system, which heavily relies on mechanical devices. Gas is assumed to drive forward from the muzzle, so the muzzle cap moves forward. That motion must then be converted to rearward force to operate the mechanism, so a seesaw linkage is incorporated to provide that. This is a very natural progression of design, but results in a very complex and cumbersome design.

Pure gas trap weapons, on the other hand, utilize the less intuitive principle of gas flow, where gas can be tapped against a stationary muzzle cap and produce rearward force directly, operating a purely rearward-moving piston, no linkage required.

To illustrate how these two mechanisms are different, YouTuber Bloke on the Range took to the blackboard:

The Bang-type system was not widely used; only the earlier designs from the Dane Søren Bang utilized the principle, as well as a handful of other experimentals like the obscure Chinese Liu rifle. The gas trap system with fixed muzzle cap, however, got further: The early models of the US M1 Garand used the mechanism, as did the German G.41(W) and G.41(M) selfloading rifles.

Readers should note that so far as I know, the simpler gas trap mechanism is a direct development of Bang’s forward moving gas cap mechanism. In fact, the earliest rifle I know of that used the gas trap system was Bang’s own 1920 model rifle!