Police recovered a dozen rifles outfitted with bump stocks in the Las Vegas hotel room from which a gunman carried out the deadliest mass shooting in modern American history, the Associated Press reports.

A bump-fire stock is a device that harnesses the recoil of a semiautomatic firearm to fire several shots in succession, mimicking automatic gunfire.

In 2015, The Trace’s Alex Yablon broke down the devices’ functionality. Yablon focused on a specific model, the Bump Fire Stock, which at the time sold for about $100. Another popular maker of the devices goes by the brand name Slide Fire.

The Bump Fire stock doesn’t convert semiautomatic rifles to true automatic fire. Rather, it provides an effective means of engaging a gun’s trigger extremely quickly. Instead of pulling back the trigger to fire, the user places his or her finger slightly in front of the trigger and pushes the whole gun forward with steady pressure. The trigger hits the finger and the round goes off. Recoil pushes the gun back, but the shooter’s forward pressure immediately returns the trigger back to the finger, and so the gun fires off another round faster than the blink of an eye.

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Here’s what that looks like in practice:

And here’s a video showing a stock from Slide Fire Solutions:


Fully automatic weapons are strictly regulated under federal law, and have been out of production in the United States since 1986. But the federal government has green lit bump-fire stocks for lawful sales. Bump Fire System’s website includes a letter of approval from the Bureau of Alcohol, Tobacco, Firearms and Explosives, which examined the product before it entered the market in 2012. The devices, as Yablon writes, are just this side of legal.

Lawmakers have attempted to crack down on devices designed to increase a semiautomatic gun’s rate of fire. In 2013, after a gunman murdered 20 children and six educators at the Sandy Hook Elementary School in Newtown, Connecticut, Senator Dianne Feinstein of California proposed a law that would have banned bump-fire stocks and similar devices. The legislation did not get a vote.