I read it was discarded in the theater. He put the magazine in the picture in the gun, but since it was jammed, it was a no-go.
So it’s entirely likely that because of his overzealous want to pack as many rounds into a magazine as possible, he actually stopped his spree short. If that’s the case, he walked out of the theater after he emptied the shotgun, the pistol, and couldn’t fire the rifle any longer.
I guess we can thank God that he was a Liberal idiot who had no desire to actually learn how to fire or maintain that rifle.