For one, there are a lot of Bushmaster AR-15s out there. It's one of the most popular sporting guns, noted Time's Madison Grey. Lanza didn't purchase his at a store, but rather, his mother, a "gun enthusiast," had owned the guns, which is reportedly how Lanza got his hands on them. It is still not clear where Spangler got his Bushmaster. As a felon, it was illegal for Spangler to possess a firearm. So he must've gotten his hands on one already purchased. Between 2000 and 2010, the company sold over 2 million of the Bushmaster make. 1.5 million AR-15s have been sold in the last five years alone, according to Guns and Ammo. Also a testament to their wide popularity, the guns started flying off shelves after Newtown.
Besides the wide availability, the gun's lethality might give some insight into why these types of shooters seek to get their hands on one. Though the Bushmaster describes itself as a semi-automatic, dispensing only one bullet at a time, the civilian make of an M-16 can shoot lots of bullets very quickly. YouTube even has some tricks for turning a "semi-auto into a full-auto machine using a household rubberband!"
The Bushmaster wasn't the only weapon used in either of these shootings. It is the one the two had in common. In addition Spangler also got his hands on a .38-caliber revolver and a 12-gauge shotgun, according to the Associated Press. Lanza also had two other guns, a Glock 10 mm and a Sig Sauer 9 mm.
In addition to the weapon, police found a haunting note left by Spangler saying the following. "I still have to get ready to see how much of the neighborhood I can burn down and do what I like doing best – killing people."