He had multiple rifles and 2 sets of tripods as per initial reports.

My theory is he used AR-15's (available to civilians but on semi-auto only) modded to fire full auto and had drum magazines. Since the AR-15 will probably overheat on full auto after a couple hundred rounds (someone correct me on this), he probably just used a different gun as soon as the clip ran out (no reloading necessary).

A fully automatic assault rifle on a tripod aimed at thousands of concert goers could have killed a lot more. They were lucky this nutcase didn't get any sort of gunnery training in the military.