A gun without ammo can't be fired. There's no indication that anyone had any ammunition for those guns anywhere on the grounds; the statement says he had the ammunition at home. It wasn't in a broom closet.
plus, there seems to be two eyewitness accounts in the locker room stating that Javaris then got his own gun, and loaded ammo into it:
http://www.washingtonpost.com/wp-dyn/content/article/2010/01/06/AR2010010605167.html