The gunman who unleashed hundreds of rounds of gunfire on a crowd of concertgoers in Las Vegas attached a “bump-stock” to two weapons, converting semi-automatic firearms into fully automatic ones, the Associated Press reports. The devices have attracted scrutiny in recent years from authorities. Sen. Dianne Feinstein (D-CA) has long railed against them. Several years ago, she said she was concerned about the emergence of new technologies that could retrofit firearms to make them fully automatic. “This replacement shoulder stock turns a semi-automatic rifle into a weapon that can fire at a rate of 400 to 800 rounds per minute,” she said. A semi-automatic weapon requires one trigger pull for each round fired. With a fully automatic firearm, one trigger pull can unleash continuous rounds until the magazine is empty.
The U.S. has curbed the purchasing of fully automatic weapons since the 1930s. In 1986, the National Firearms Act was amended to prohibit the transfer or possession of machine guns by civilians, with an exception for guns previously manufactured and registered. Attempts to design retrofits failed until recent years, when bump stocks came on the market. The device replaces the gun’s shoulder rest, with a “support step” that covers the trigger opening. By holding the pistol grip with one hand and pushing forward on the barrel with the other, the shooter’s finger comes in contact with the trigger. The recoil causes the gun to “bump” the trigger. Technically, the finger is pulling the trigger for each round fired, keeping the weapon a legal semi-automatic. Las Vegas shooter Stephen Paddock had 23 guns in his hotel room.