Page "Semi-automatic rifle" Paragraph 10

The second method of operation, called direct impingement, is somewhat simplified.

Here, the tube carrying the high pressure gas ported from the barrel emerges inside the action of the rifle, where it mates up with a " gas key " which is integral to the bolt carrier.

Thus, the bolt carrier itself functions as a piston as in the French MAS 49 and 49-56 rifles.

The AR-15 and its military versions, the automatic M16 and M4 Carbine, are the most common and well-known weapons which utilize this system.