So it's become fashionable to claim that the Eugene Stoner direct impingement gas system used in the .308/7.62 AR-10 and .223/5.56 AR-15 isn't actually direct impingement due to there being an integrated piston in it. Unlike the Ljungmann AG42 and French MAS 44, 49 and 49/56, which... erm... also have an integrated piston.... This point of view is largely supported based on an irrelevant single phrase in the original US patent on the system combined with a logical fallacy.

Tin hats on, chaps, we're giving this the full BotR Nerdathon!