It's extremely hard to hit someone going more than 600 units per second, even if they're facing you with their back. The other day I tried playing on WTF's pro CTF server (I was mostly used as cannon fodder) and I saw some people achieving insane speeds while circle jumping. Forget hitting that with any gun. They have the kind of players that go 1000mph past the corner where you're hiding on Riyadh, power slide, turn around, and kill you with two perfect AK shots as you empty your entire magazine into them without hitting ONCE ;p