According to the Associated Press, the gunman in Las Vegas used a bump stock. The legality of bump stocks is interesting, because they are legal or illegal depending. What a bump stock does is provide a channel for the receiver of the firearm to ride back and forth in. You place your finger in the trigger guard, then pull the receiver forward in the stock onto your finger which depresses the trigger and fires the rifle. The recoil forces the receiver back in the stock so that your finger releases the trigger and it resets. Pull forward again and you start the cycle over again. If you apply continued forward pressure, with a bit of finesse you can use the recoil of the gun to achieve a cyclic rate of fire very close to fully automatic fire.
ATF has concluded this is legal, because the law defines a machine gun as a firearm which fires two or more shots “with a single function of the trigger,” and in this case the shooter’s finger is pulling the trigger for each shot. You can do bump firing without a sliding stock, of course, but it’s more difficult to achieve.
However, if you put a spring in a bump stock, such that the spring pushes the receiver back into your finger, this is considered an illegal conversion and you’ll go to jail. The ATF considers the spring to be a machine gun. It’s a device pushing the trigger into the finger, not the shooter. So the shooter is only doing one function of the trigger, while the firearm is continuously firing because of the action of the spring. That is, legally, a machine gun.
I’ll be completely honest with you, if we could get SHARE and National Reciprocity through, I’d trade making bump stocks machine guns. But I don’t think we’ll have to do that. Our opponents will overreach, they will try to ban semi-automatics, our people will arise and push back, and they will end up with nothing in the end.