If the gun runs out of bullets in 45 seconds, you could probably get away with pretty small batteries. The main thing it's doing is spinning a barrel, which you could probably do with a hand-held drill. The trick is that you don't want to wait while the barrel spins up, so you want enough power to get it up to full speed very quickly, which means a lot of initial power drawn from the battery, then a lot less to keep the barrels spinning.