How a bump stock modifies a semi-automatic firearm

An agent in charge of the ATF investigation into the mass shooting in Las Vegas said the shooter had a dozen bump stocks in his hotel room.

Modifying a firearm with those particular stocks is legal in Nevada, even though it alters a semi-automatic weapon so that it mimics a rapid-fire rifle.
“I don't know of any other legal accessory or device that does that other than a bump fire. And anything that modifies the trigger group or the fire control group, anything that does that is illegal,” said Kent Morrison security officer instructor and founder of BSG Security Services.

Bump stocks use the gun's recoil to help fire rounds more rapidly. 

“The full auto is designed to fire more than one shot with a single pull of a trigger, the bump fire is designed to simulate that rapid fire by how it makes the gun jump around and makes your finger manipulate the trigger multiple times very quickly,” Morrison said. 

There are some big differences between a semi-automatic weapon modified with a bump stock versus a fully automatic firearm. A full-auto fires multiple times with one pull of the trigger, whereas a semi-automatic firearm modified with a bump stock still requires the trigger to be pulled once for each round fired.

“You're never going to get any kind of real accuracy with it, as you can with either a semi-automatic rifle where you're going to get a good sight picture and a good sight alignment or a full auto that is designed to be shot in that fashion,” said Morrison. 

Because the bump stock still requires the trigger to be pulled for each shot, the ATF considers it legal. However, some states have their own laws banning the stocks.

Those familiar with the modifications tell us there is really no purpose for them besides novelty.
“I will say it definitely increases how fast you can shoot a semi-automatic. It remains a semi-automatic firearm, it doesn't change the nature of how the gun works mechanically,” Morrison said.

The ATF could make bump fire stocks illegal by changing the definition of a machine gun.

Lawmakers could also act, but a bill proposed in 2013 banning the stocks did not make it through Congress.