Two legal, off-the-shelf devices could have been used to recreate a fully-automatic rate of fire, increasing the speed by hundreds of rounds per minute.
One, known as a “trigger crank” or “gat crank” bolts onto the trigger guard of a semi-automatic rifle. The shooter then rotates the crank, which usually depresses the trigger three times per rotation, Bernstein said.
The trigger crank’s sell online for about $40-$50 and can be purchased by mail.
“They really shouldn’t be legal — anybody with an AR-15 can bolt one on and crank out rounds as fast as a fully-automatic without a class III machine gun license,” Bernstein said, adding that he doesn’t sell the devices at his shop in Colorado.
Another modification to a semi-automatic weapon that could have been used in the shooting is what’s called a “bump stock.”
The device modifies the stock of the gun so the recoil helps fire rounds in rapid succession. The bump stock is also legal in the U.S.
“You can take an M-16 and it fires about the same as a bump fire—it goes with your body back and forth,” Bernstein said.
The shooter could have used semi-automatic weapons with legal modifications, or a fully-automatic weapon with, or without a permit, said Mike McLively, a staff attorney at the Law Center to Prevent Gun Violence.
“The only issue with the modifications is you lose accuracy, but of course if you’re aiming at tens of thousands of people in a crowd, accuracy doesn’t matter as much,” McLively said. “If it turns out a modification was used, I think it’ll start a conversation about whether they should be legal. People think it’s fun to shoot a lot of rounds—but you have to weigh that against the 500 people injured and nearly 60 people killed.”