Police believe the Las Vegas gunman used a “bump stock” to turn a concert into carnage. On Thursday, Republicans and the National Rifle Association agreed—the pieces of equipment should be regulated.

The NRA released a statement saying it’s “calling on the Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE) to immediately review whether these devices comply with federal law."

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Stephen Paddock had bump stocks attached to 12 weapons recovered from his hotel room, according to CBS News.

So, what is a bump stock?

To understand a bump stock, you have to understand the legal definition of a machine gun, explained attorney John Pierce, who’s well-known within the firearms community and focuses on gun rights.

The ATF, under the National Firearms ACT , defines a machine gun as: “Any weapon which shoots, is designed to shoot, or can be readily restored to shoot automatically more than one shot without manual reloading by a single function of the trigger.”

In plain English, a machine gun is a firearm that can rapidly fire as long as the trigger is pressed.

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“A Bump Fire Stock is one of a number of technologies that allows a shooter to increase the rate of fire of a semi-automatic firearm to closer to the rate of fire that you would get from a fully automatic firearm,” Pierce explained.

“What it does is it uses the recoil from the shot that was just made,” Pierce continued.

A Texas gun store employee described it to CBS News like this—the modified stock allows the firearm to slide. As you fire, “it vibrates,” said Tom Mannewitz, who continued, “and that’s causing your finger to keep bumping the trigger—that’s why it’s called a bump fire stock.”

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How are they legal?

Pierce said under the Obama Administration, the ATF was asked to rule on the bump stock modification, which determined it is not a machine gun.

Pierce explained the decision was made "because the semi-automatic firearm that is modified by the bump fire stock only fires a single round with every pull of the trigger.”

Pierce also said it’s not just bump stocks—there are several other types of modifications out there that can produce more rapid fire, even some that modify the trigger.