Along with the 23 guns that police officers found in Vegas shooter Stephen Paddock's Mandalay
Bay hotel room, officials also found two "bump stocks." These devices, which are legal, use a semi-automatic weapon's recoil to allow it to fire repeatedly at a rate closer to that of a fully-automatic weapon.
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Bump stocks are simple pieces of equipment that replace the stock of a rifle and add a small "support step" in front of the trigger. The shooter rests his finger on this step and pulls forward on the barrel or forward grip to press the trigger against his finger. The recoil of the shot then propels the rifle backwards into a gap in stationary stock where the loose fit gives the rifle freedom to bounce forward. This, along with sustained forward pressure on the rifle, has the effect of 'bumping' the trigger back into the shooter's unmoving finger. So long as a shooter maintains forward pressure, the rifle will continue to fire at a rate much faster than could be accomplished with even the quickest possible series of manual trigger pulls.
Two of the main manufacturers of bump stocks—Bump Fire Systems and Slide Fire—have posted letters from the Federal Bureau of Alcohol, Tobacco, Firearms, and Explosives, which declare these devices as legal in large part because they "[have] no automatically functioning mechanical parts or springs and [perform] no automatic function when installed."
They do, however, have the practical effect of drastically increasing rate of fire. As California Sen. Dianne Feinstein told the AP, "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."
Officials are still investigating whether the two bump stocks found in the room were actually used along with the rifles during the massacre.
http://www.popularmechanics.com/technology/news/a28479/vegas-shooter-bump-stock/
