ATLANTA — Las Vegas gunman Stephen Paddock possessed a little-known device called a “bump stock” that was not widely sold – until now.

Originally created with the idea of making it easier for people with disabilities to shoot a gun, the attachments allow a semi-automatic rifle to mimic a fully automatic weapon by unleashing an entire large magazine in seconds. Now the deadliest mass shooting in modern U.S. history has drawn attention to the devices, which critics say flout federal restrictions on automatic guns.

Bump stocks such as this one next to a disassembled .22-caliber rifle have been around since the government approved their sale in 2010. Associated Press/Allen Breed

The stocks have been around for less than a decade. The government gave its seal of approval to selling them in 2010 after concluding that they did not violate federal law.

The device basically replaces the gun’s shoulder rest, with a “support step” that covers the trigger opening. By holding the pistol grip with one hand and pushing forward on the barrel with the other, the shooter’s finger comes in contact with the trigger. The recoil causes the gun to buck back and forth, “bumping” the trigger.

Technically, that means the finger is pulling the trigger for each round fired, keeping the weapon a legal semi-automatic. The rapid fire does not necessarily make the weapon any more lethal – much of that would be dependent on the type of ammunition used. But it does allow the person firing the weapon to get off more shots more quickly.

It’s unclear how many have been sold. The industry leader, Slide Force, did not return messages seeking comment. But the Abilene, Texas, company’s Facebook page is filled with videos extolling its features, including one in which a woman gushed, “It’s so easy because once you slid it forward and leaned into it, it just fires.” In another video, a man fires off 58 rounds to celebrate his 58th birthday in just 12 seconds.

Sales for firearms or specific accessories seem to jump after every high-profile shooting. That will likely happen again with bump stocks, said Dr. Garen Wintemute, director of the Violence Prevention Research Program at the University of California, Davis.

“People will go, ‘Oh geez, I should get one of those.’ The other is that people will be concerned about efforts to ban them,” Wintemute said.

Manufacturers tout the stocks, which typically sell for less than $200, as offering a simple and affordable alternative to automatic weapons without the hassle of a rigorous background check and other restrictions.

Ed Turner, a former police officer who owns a gun shop in Stockbridge, Georgia, said he’s already seeing a run on bump stocks since the shooting. He said he would be surprised if he had sold two of them in the past decade, but now he’s unable to find any available, even from wholesalers.

While the stocks allow a gun to quickly spray bullets, gun experts say they also create such a jolt that accuracy is affected. That may not matter to gun owners who just want the thrill of shooting with one, or for those bent on destruction. Stephen Paddock, the 64-year-old gunman, fired hundreds of rounds indiscriminately from his 32nd-floor room at the Mandalay Bay Hotel and Casino on a music festival outside.

He had 23 guns in the room. Authorities found two bump stocks in the room, two officials familiar with the investigation told The Associated Press.

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