Ben Bazooka said:
As one having fired with an assault rifle plenty of times, the sounds aren't right there realistically. But it's not as much about sounding realistic than sounding good.
I don't know about that, I happen to like the way guns sound in real life so with that you can't really go wrong.

(apart from the fact that if you don't wear some sort of ear protection you'll soon get a headache and dulled hearing from listening to gunshots for too long).
My main gripe with the realism is the fact that guns are FAR from loud enough in videogames to actually sound good and somewhat realistic. Heck even a puny .22 gunshot going of in your hand makes a ridiculously loud report, even if you are having your ear stuffed with cotton.
So in my opinion, most games would benefit from seriously amping up the volume when the guns start blazing. A gun that barely makes any sound at all (without a silencer attached of course) is just so embarassing it's borderline emasculating.
But back on topic: The best gun sound effect of all time originally comes from a movie and not a game (although the sound effect has been used in games since) and that is the report of a pulse carbine from the movie Aliens 2, firing on full auto bursts. It just sounds so... RIGHT, for some reason. XD
A close runner up would be the way that Dante's pistols Ebony and Ivory makes in Devil May Cry (the first game for PS2). It doesn't matter how many shots you fire off, i's just plain addictive to hear.