The trigger crank's sell online for about $40-$50 and can be purchased by mail.
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"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.
It's also possible the shooter had fully-automatic weapons, which are only legal with a special permit that requires fingerprinting and federal approval.
Clark County Sheriff Joe Lombardo identified the shooter as Stephen Paddock, 64, who the sheriff said was found dead by officers after they stormed into his hotel room on the 32nd floor of the Mandalay Bay Resort and Casino.
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Kristen Rand, legislative director for the Violence Policy Center, says there are a variety of tools on the market for speeding up the number of rounds fired by semiautomatic weapons.
Rand said the Bureau of Alcohol, Tobacco, Firearms and Explosives reviews devices individually to determine whether they function as a machine gun, or whether the action pulls the trigger independently for each round. The ATF has said that many of these function, technically, as a semi-automatic weapon — making them legal.
The devices have been around for a long time, but have become more popular recently, she said. There's a lot of questions about how effective they really are, she said.
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“It’s just a gimmick," she said. "It’s just a gimmick to make your Youtube video and show people how fast you’re shooting your gun.”
Another option to convert a weapon to fire continually is with purchased parts or through gunsmithing — an illegal process.