Well since 12 x 30 is only 360, that's 360 rounds per game and I've only ever got through all 12 mags once.
Once I played a 3 hour game and still had 2 mags left at the end of the game. I just use semi practically all the time, auto is exclusively for long range moving targets that are a direct threat to me, my team or the objective; clearing bunkers and rooms; and for annoying assholes who don't take their hits.
I imagine I use a hell of a lot less ammo than most people, especially considering all the hi-cap heros have more ammo in one mag than I do in all of mine.
But yah =] You could say I was very conservative. But if you did, by association, I'd have to kill myself lol.