In part:
"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."
https:// www.popularmechanics.com/technology/news/a28479/vegas-shooter-bump-stock/
A concession to the gun-grabbing democrats ??