There are three different types of operating systems being discussed here:
1. Op-rod driven guns (which can be broken down into floating interrupted, or direct attach pistons)
2. Direct Impingement guns, like the MAS-49 and Ljungman Ag m/42
3. Internal Expansion Stoner System
This is why I don't refer to the op rod driven guns as piston guns, even though they all have pistons. Simply ID'ing them as piston operated isn't detailed enough to understand the method of operation.
All of them are gas operated.
All of them have gas directly acting on some mechanical part to cause inertia on the bolt carrier. If you get loose with the terminology, what you are talking about becomes less precise, and more vague.
The primary difference between a true Direct Impingement gun and the Stoner Internal Expansion system is that gas acts on a disconnected cup on top of the bolt carrier with the MAS and Ljungman.
With the Stoner, it flows down into a chamber formed by the stationary piston, gas rings, and bolt carrier bore and bolt tail aperture. I made a drawing to illustrate this, since it is probably the most overlooked aspect of how the Stoner system really works.
The difference is in how the gas acts on the operating parts. In the Ljungman, it directly impinges on the internal surfaces of the cup, forcing the carrier rearward. The problem with the MAS and Ljunman are that gas and debris can only vent out of the front, and onto the bolt face, right in the action. Stoner routed the gas through the carrier, originally on the side of the early tool room AR10's, with vent holes on the ejection port side, so once the carrier had moved enough rearwards, it allowed timed venting past the gas rings.
In the Stoner system, it expands and is trapped in all directions, which is why Stoner described it as such in the patent:
If you want to use the word "direct" for the Stoner system, then it would be appropriate to call it a Direct Internal Expansion system, not Direct Impingement. You would also call the AK a Direct Gas Piston system.
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