Officials say the gunman who unleashed hundreds of rounds of gunfire on a crowd of concertgoers in Las Vegas had two "bump-stocks" that could have converted semi-automatic firearms into fully automatic ones.
The devices have attracted scrutiny in recent years from authorities.
The device basically 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 buck back and forth, "bumping" the trigger.
Technically, that means 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.