On Tuesday, President Trump said he directed the Justice Department to ban the sale of gun modifications that simulate automatic fire, including bump stocks, in the wake of a high school shooting in Florida that left 17 people dead on Feb. 14.

A law enforcement official familiar with the investigation said Nikolas Cruz did not use a bump stock modification in the attack, but calls for banning them were renewed following the killings.

The device was used with deadly efficiency in Las Vegas on Oct 1. Stephen Paddock, 64, smashed the windows of his hotel room on the 32nd floor of the Mandalay Bay Resort and Casino and fired at a music festival crowd below, leaving 58 people dead and hundreds injured. Paddock killed himself before police could reach him.

Audio of the gunfire suggested the sound of an automatic weapon. Law enforcement officials later said that at least a dozen firearms Paddock had with him were modified with bump stocks to fire in a way that mimics, but does not replicate, automatic weapons.

How does it work?

A bump stock is a piece of plastic or metal molded to the lower end of a rifle. The device allows a shooter to fire dozens of rounds in seconds by harnessing the gun’s natural recoil. A rifle with this type of mechanism is optimal with a high-capacity magazine that can hold between 60 and 100 rounds and a hand grip that allows a shooter to push the rifle away from the body to bounce, or bump, the weapon into the trigger finger.

When these are combined, the weapon can shoot large amounts of automatic-like fire without much concern for accuracy, as the recoil from simulated automatic fire would make it difficult to hit specific targets at a long range. But accuracy was not an interest for Paddock, whose elevated position and fire into a dense crowd overcame the disadvantage of inaccurate targeting.

Are they legal?

Yes, but possibly not for long if the Justice Department implements a ban on the sale and manufacture of the stocks. It is unclear whether existing devices would be grandfathered into any future law.

Bump stocks have been around since 2001, and the government has not regulated them. That’s because the modifications do not include alterations to the weapon’s mechanical components.

Slide Fire, one of the companies that first began selling the device, announced days after the Las Vegas killings that they were sold out of bump stocks “due to extreme high demand.”

How do bump stocks play into the gun-control debate?

At first, the National Rifle Association was silent on the issue of bump stocks. But days after the Las Vegas massacre, the organization announced that it would support regulation of the device — as did the White House.

Sen. Dianne Feinstein (D-Calif.), a fierce advocate of gun control, introduced legislation in October that would ban the import, sale, manufacturing, transfer and possession of bump stocks and similar inventions.

“The only reason to modify a gun is to kill as many people as possible in as short a time as possible,” Feinstein said.

Republican lawmakers in Congress also backed legislation to ban bump stocks, a rare moment of GOP politicians calling for restrictive measures on firearm accessories.

“I didn’t know what a bump stock was until this week,” House Speaker Paul D. Ryan (R-Wis.) said in October. “A lot of us are coming up to speed. … Having said that, fully automatic weapons have been outlawed for many, many years. This seems to be a way of going around that, so obviously we need to look at how we can tighten up the compliance with this law so that fully automatic weapons are banned.”

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