Plot hole: Aside from the fact that you can't have a semi-automatic rifle delivered to your doorstep (except to an authorized dealer), there's also the fact that the weapon apparently was shipped with a full magazine. This is ridiculous. All the boy did was take the mag from the box to the gun and it started firing right away. One can assume they already had the ammo and filled the mag inside the house. This is countered by the fact that when he takes the mag out of the box, it is empty. Freeze-frame confirms there is not a single cartridge in the magazine before it is inserted into the weapon.