Here is
Here is a video, and I'm sure I can post many others as well, but pay attention to what he says at the very beginning. Possibly you will hear "direct impingement":
If it uses a BCG, it's direct impingement. There is a difference between "acts like a piston" and "gas piston".How is it an argument? Its the first paragraph of a patent application that explains how an AR works, by someone who actually knows what they are talking about.
No all pistons are long stroke like the M1/AK. Some, like the AR15, AR180, SVT 40, are short stroke. The AR15 just has some of those parts built into other parts, doing 2 things at once.
Here is a video, and I'm sure I can post many others as well, but pay attention to what he says at the very beginning. Possibly you will hear "direct impingement":
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