The school shooter in Broward County, Florida on Wednesday used an Armalite-style rifle to carry out the attack.
Investigators said the gunman, Nikolas Cruz, had several magazines as well.
SPECIAL SECTION: Shooting at Broward County school
Alex Shkop, the owner of Guns and Range Training Center in West Palm Beach, said the AR-15, is one of the most widely-owned guns in the world.
Even though it's difficult to conceal, it's probably the only gun that Cruz could have gotten his hands on legally.
“You cannot purchase a handgun until you are 21,” said Shkop. “I believe the reason he used an AR-15 is because he is under 21 and you cannot purchase a firearm other than a long-gun legally. I don't know if he purchased it legally, if it's his, because it's a really hard hand gun to smuggle, and especially entering a school.
Shkop says while it may be easier to learn how to shoot an AR-15, it's very difficult to sneak it inside a public place.