The gunfire that erupted out a window of the thirty-second floor of the Mandalay Bay hotel in Las Vegas came in long, high-rate-of-fire bursts. In the immediate aftermath of the attack that has so far left 59 dead and over 500 injured, many wondered if the killer had used an automatic weapon. It’s not entirely impossible: automatic firearms, while heavily restricted, are available in some states. But firearms experts and evidence from the scene suggest that the killer may have instead used a legal attachment known as a ‘bump fire stock’ to turn much more readily available semi-automatic weapons into something capable of much greater destruction.
To understand what a bump fire stock does, we first need to understand some simple differences between firearms. In automatic weapons, the shooter only needs to hold down the trigger and the gun will continuously fire until the magazine is empty. A semi-automatic, on the other hand, requires the shooter to pull the trigger for every bullet that they fire. A semi-automatic’s rate of fire is limited by how quickly the shooter can pull the trigger, which is generally considerably slower - it’s been suggested that semi-autos can achieve a practical rate of fire of 90-120 rounds per minute, while a full automatic can be anywhere from 600-1,200 rounds per minute.
A bump fire stock, though, essentially gives a semi-automatic the capacity to fire like an automatic. The stock, which replaces the gun’s factory stock and pistol grip, is attached to the rifle in such a way that when the rifle fires the recoil forces the ‘front’ of the gun back into the stock and then back out again. The shooter rests their finger on the modified trigger guard, and as the gun bounces forward off the back of the stock the trigger is depressed by the finger again, causing the process to start over. Swapping out the need to pull the trigger manually with springs and recoil makes it all happen very fast. Depending on the gun and the round, a semi-automatic may be able to fire anywhere from 400-800 rounds per minute with a bump fire stock.
Law enforcement officials involved in the Las Vegas shooting case told Associated Press that they found two bump stocks in the gunman’s room, along with 23 guns. It’s not known at this time if the bump fire stocks were actually used in the shooting, nor is it known if any of the 23 guns are true automatics.
But even before the stocks were discovered, some experts were suggesting they were involved. Firearms expert, instructor and author Massad Ayoob told The Guardian that he suspected a bump fire system or ‘hellfire’ attachment, which works in a similar way but with a hand crank depressing the trigger, might be involved because the gunshots “did not sound as consistent” as he would expect from an M16 or AK47, two of the more popular fully automatic weapons available in the United States.
“The pace of fire is a little bit erratic. At one point it’s slower than it is at another point,” he said.
Whether or not the gunman used such a system, it’s likely to thrust such firearm modifications back into the spotlight. Currently, bump fire stocks are legal because the trigger is being depressed each time the weapon fires, ensuring that the weapon retains semi-automatic status. That, though, may soon change.