I never heard of bumpstocks until this incident. I don't really see why they allow civilians to have that capability, even in this world?
http://uproxx.com/news/what-is-bump-stock-legal-stephen-paddock/
In a bump stock, there’s a gap in the stationary stock allowing the gun’s recoil to drive it backward. There’s also a “support step” where the shooter can rest their finger. Then they simply pull the gun forward. When it fires, the semiautomatic action engages, reloads, and the trigger hits the shooter’s finger, firing again. The gun is “bumping” against the stock, and thus is automatic.
As to why bump stocks are legal, under the law, you cannot convert a semiautomatic rifle to a fully automatic one, yes, but that’s not, technically, what a bump stock is doing. Swap out the stock, a relatively simple matter for an experienced shooter, and the gun is a semiautomatic. An automatic weapon is defined under the law as one with an internal automatic mechanism. Since the stock isn’t an internal mechanism, it’s legal.
http://www.cnn.com/2017/10/04/us/bump-stock-las-vegas-shooting/index.html
Are they legal?
While automatic weapons are tightly regulated in the US, the use of bump-fire stocks in semi-automatic weapons is legal.
Bump-fire stocks allow semi-automatic weapons to simulate automatic fire but "do not actually alter the firearm to fire automatically, making them legal under current federal law," Snyder said.
But some believe bump-fire stocks, while legal, violate the spirit of gun restrictions.
"This is something that works around and circumvents our laws," Gagliano told CNN's Chris Cuomo on "New Day." "You're not mechanically modifying a weapon, but you're putting an aftermarket product on that allows you to lay down ... suppressive fire."