Gruesome videos from the Las Vegas shooting revealed the attacker was able to fire at a rate comparable to an automatic weapon, which are illegal to sell in the United States.
The shooter, Stephen Paddock, 64, had many of his rifles outfitted with a bump fire stock, a modified part that allows for rapid fire while dodging the laws banning automatic weapons.
That way, Paddock was able to fire hundreds of rounds in some 10 minutes from his 32nd floor hotel room, killing 59 and injuring over 500 at a crowded country music concert down below.
Semi-automatic rifles, which can be legally purchased in America, allow only one shot to be fired with each pressing of the trigger.
Bump fire stock is a modified stock (the part of the gun set against the shoulder) that allows the shooter to press the trigger repeatedly at a high rate. It includes a grip that keeps the trigger finger at a static position. The shooter fires the gun by pushing the barrel forward, thus pressing the trigger against the static finger. As the round is fired, the recoil from the shot sends the body of the gun back, where it slides about an inch inside the modified stock. As the shooter keeps pushing the barrel forward, the gun slides out of the stock about one inch and the trigger is pressed against the finger again.
Multiple video demonstrations online show the bump fire stock can make a semi-automatic rifle achieve a rate of fire similar to an automatic one.
One manufacturer of bump fire stocks, Slide Fire, states on its website it submitted the part to the Bureau of Alcohol, Tobacco, Firearms, and Explosives and had it approved.
The argument for the legality of bump fire stocks is that the rifle itself remains semi-automatic. The stock only helps the shooter press the trigger much faster.
Some users have complained online they have had trouble making the bump fire stock work properly.
Shortly after the news of the Las Vegas shooting spread, many people with gun experience have noticed the unusual firing rate, which seemed to be sometimes slower and sometimes faster. This firing rate inconsistency is different from an automatic rifle, but could be explained by the use of a bump fire stock.
The shooter also used 60 and 100-round magazines—some of the largest available.
Police said he had 22 rifles and a handgun in his hotel room. Another 19 guns were found in his Mesquite, Nevada, home.
Police said Paddock shot himself in the room. His motive remains unclear. Police hope to get some answers from his girlfriend Marilou Danley, who was in the Philippines, her home country, during the shooting and returned to Los Angeles on Tuesday, Oct. 3.