If you're a fan of the AR rifle or thinking about owning one, here's a video where two guys square off in a friendly argument about whether the original design that uses D.I. (direct impingement) is better then a similar piston-operated AR.

The piston guns are favored by some, including US Special forces/black ops etc. who use whatever rifles they want to, and run the gamut from the HK 416 to the XM8. The problem with the DI system is this: fouling. when the rifle is fired the gases blow through a tiny hole in the barrel, back through a stainless steel gas tube and blow back the bolt. No ammunition ever burns truly "clean" so this means the system is prone to fouling and possible jamming in extreme conditions i.e. every fire-fight that ever happens anyplace the rifle is in use.

The army has put out a bid for a replacement for the government issue, DI M16/M4 weapons and after continuous service and 3 upgrades since 1965, it's certain whatever the army chooses will be piston operated and not DI.

Where else can you see two gun-wielding guys arguing and it not even cross your mind they are going to point them at each other at some point ha ha.