The Las Vegas shooter attached “bump stocks” on semi automatic rifles which enables the shooter to rapidly fire rounds at a speed that rivals fully automatic machine guns.
Lawmakers are eager to ban the stocks, but the attachments are so seldom used that it is hard to imagine any such ban would make even a fraction of a difference in the violent crime rate. It is mostly just a way for lawmakers and the NRA to crow about having done something.
The New York Times explained:
The bump stock is not banned under federal law even though it allows a weapon to fire at nearly the rate of a machine gun without technically converting it to a fully automatic firearm. (It is illegal for private citizens to possess fully automatic firearms manufactured after May 19, 1986; ownership of earlier models requires a federal license.)
One of the bump-stock manufacturers, SlideFire, back in 2010 submitted a letter saying that they are legal in part because they don’t make the gun fully automatic, saying they “[have] no automatically functioning mechanical parts or springs and [perform] no automatic function when installed,” and that it is a firearm part and not regulated under the Gun Control Act or the National Firearms Act.