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.
While this is common knowledge to most, this movie was inspired by the events of Columbine in 1999. Some scenes mimick the real-life massacre almost exactly. For instance, they planned to kill many more than they actually were able to because of the failure of their improvised explosive devices. The real killers had planned to murder at least 600 people, though it is not mentioned in this film the scope of the plan. Another, more obvious, link is the use of the Tec-9 assault pistol. One can also link the use of settings like the Library and the Cafeteria to the media coverage of the real event.Link: http://en.wikipedia.org/wiki/Columbine_High_School_massacreFirearms.