120/2 = 60
That's 60 men you can kill if you double tap them all, and don't miss once. Not exactly a small number. That is, until you take into account that your average grunt probably puts about 8 rounds into anything that either looks like a target, or his mates decide to shoot at.
If it were up to me, it'd be 300 rounds. And if you're too piss weak to carry 10 mags, then go away.