Pretty much. The military maintains a large supply of captured enemy ammo. Keep in mind, we are equipping Iraqi security forces with 7.62x39. As far as soldiers getting their asses chewed for using captured weaponry, that would be up to individual commands. Some commands might frown on it while others condone it. If you see your officers patrolling with Kalashnikovs, that is a good sign that they aren't going to be too bothered if you brandish one yourself.
Using captured weapons goes way back. I've seen photos of American soldiers in WWII using German MP-40s.