Well, you can
use 12g's with it right off the bat but you aren't going to get too many shots. You'd also need to get a 12g quick changer. They cost about $14-$15 at most paintball shops.
I you really want to start using 12g's, you should do the mod to the striker to reduce the weight (http://www.pbreview.com/forums/showt...threadid=73889
the 7th post). That way you'll get about 35-40 shots off a 12g instead of 20-25.