AA's aiming system is utterly bogus.
I ran round behind a hill on Mountain Pass and crept up behind a sniper. He was standing in full view and I was about five metres away. Knowing I'd have to be careful of lag (on a US Army server) I carefully centred the crosshair on his head and pulled the trigger.
Nothing. No hit, on a stationary target, from behind. Emptied most of the mag into him. I don't care how bad my ping is, that shit shouldn't happen.
Some guy told me not to aim for head, don't know if that's true or not. They seem to have taken a good engine (Unreal) and mucked around something fierce with the code. I doubt I'll be spending that much time on this game.