I have stood behind a friend (let's call him X) watching him play, and like you, he had this almost uncanny ability to one-shot targets. We were in a LAN game and there was no way he was playing with any bots or exploits, since he would sometimes take over from someone else's PC when they went for a break, and suddenly this person's handle would rack up tons of kills (then someone would call out, is that you, X, on Y's PC, darn it, and we'd all start laughing).