UPDATE: The Supreme Court refused on Thursday to block a Trump administration ban on bump stocks.

Twelve of the rifles the gunman in the Las Vegas mass shooting had in his 32nd-floor hotel room were each modified with a “bump stock,” an attachment that enables a semiautomatic rifle to fire faster.

Within a week after the massacre, the National Rifle Association announced that it would support tighter restrictions on such devices.

A “bump stock” replaces a rifle’s standard stock, which is the part held against the shoulder. It frees the weapon to slide back and forth rapidly, harnessing the energy from the kickback shooters feel when the weapon fires.

By The New York Times

The stock “bumps” back and forth between the shooter’s shoulder and trigger finger, causing the rifle to rapidly fire again and again. The shooter holds his or her trigger finger in place, while maintaining forward pressure on the barrel and backward pressure on the pistol grip while firing.

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.)

“The classification of these devices depends on whether they mechanically alter the function of the firearm to fire fully automatic,” Jill Snyder, a special agent in charge at the Bureau of Alcohol, Tobacco, Firearms and Explosives, said at a news conference in Las Vegas on Tuesday. “Bump-fire stocks, while simulating automatic fire, do not actually alter the firearm to fire automatically, making them legal under current federal law.”

How Fast Is a Rifle With a Bump Stock?

Analysis of video posted on social media suggests that the gunman used rifles with rapid-fire capabilities.

This video shows 15 seconds of the attack, with constant gunshots ringing out.

Isolated, the pattern of gunshots looks like this.

Las Vegas About 90 shots in 10 seconds

Compare that with audio extracted from a video of the June 2016 Orlando nightclub shooting, in which 49 people were killed and 53 were wounded. The gunman, Omar Mateen, used at least two guns, including a semiautomatic AR-15-style assault rifle.

Orlando nightclub 24 shots in 9 seconds

In contrast, a fully automatic weapon, like this pre-1986 Colt AR-15A2, sounds different. There are no variations in the firing rate like there was in the Las Vegas shooting.