The video you see above documents a currently occurring glitch in Call of Duty: Modern Warfare 2 multiplayer for the Xbox 360. As you can see, for some as-yet-to-be-determined reason, players in some matches are receiving infinite ammo on all weapons, including heavy hitting gear like M203s and AC-130 gunships.
Judging from reports on internet message boards, the glitch is affecting 360s right now, and may be some sort of exploit that spreads to other users in a game and breaks all weapons' ammo counters.
Infinity Ward is on the case, or so IW's Robert Bowling tweets:
"Preparing an in depth #MW2 Status Report for players to stay aware of what we're addressing & how it's progressing. Lots of updates in works."
Reports are coming in that the 360 version of the game is all but broken at the moment, with just about every match "infected" with the unlimited ammo plague! I'm going into the game capture lab to check it out for myself, but maybe you've already seen this at work? What's going on out there?