'Bump-stock' devices: How Las Vegas shooter Stephen Paddock used 'souped-up semi-automatic' weapons

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The deadliest mass shooting in US historyhas thrown the spotlight on a device that turns a legal semi-automatic firearm into an even deadlier fully automatic one.

The rat-a-tat sound of Stephen Paddock's gunfire as he sprayed bullets on concertgoers from the 32nd floor of the Mandalay Bay hotel in Las Vegas prompted police at the scene to report the gunman was using an "automatic" weapon.

The term is often used to describe a fully automatic gun that can fire as many rounds as its magazine, drum or belt holds by pulling and holding the trigger.

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Those weapons have been largely outlawed for three decades, though Paddock used a device to give him "a souped-up semi".

Jill Snyder, special agent for the U.S. Bureau of Alcohol Tobacco and Firearms (ATF), said 12 of the guns found in the gunman's hotel room were fitted with so-called "bump-stocks". 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.

"There's very little difference between a souped-up semi automatic and a fully automatic," said Mike McLively, a policy expert with Americans for Responsible Solutions.

The barrage of gunfire into a crowd of 22,000 people came in extended bursts that lasted several minutes, as throngs of terrified music fans cowered desperately on the open ground, hemmed in by fellow concert-goers, while others at the edge tried to flee.

At least 59 people have died and more than 500 injured.

Graphic: How Bump-stock devices work

"From the audio, that is not someone who has a traditional semi-automatic rifle firing it in its normal condition," said David Chipman, a former Bureau of Alcohol, Tobacco, Firearms and Explosives agent who advises the gun control group Americans for Responsible Solutions.

"Either it's a machine gun or it's been modified. I don't believe a human can do that with his finger."

The purchasing of fully automatic weapons has been significantly restricted in the US since the 1930s. In 1986, the federal National Firearms Act was amended further to prohibit the transfer or possession of machine guns by civilians, with an exception for those previously manufactured and registered.

Numerous attempts to design retrofits failed until recent years when bump stocks came on the market.

California Sen. Dianne Feinstein has long railed against "bump stocks". Several years ago, she told The Associated Press she was concerned about the emergence of new technologies that could retrofit firearms to make them fully automatic.

"This replacement shoulder stock turns a semi-automatic rifle into a weapon that can fire at a rate of 400 to 800 rounds per minute," she said.

Police said they recovered a total of 42 weapons belonging to Paddock, including 23 from the hotel room and 19 at his home in Mesquite, a small desert town about an hour from Las Vegas. Some were automatic weapons or semi-automatic rifles illegally modified into fully-automatic weapons.

An employee of North Raleigh Guns demonstrates how a "bump" stock works - Credit: AP
An employee of North Raleigh Guns demonstrates how a "bump" stock works Credit: AP

It was not clear when or how Paddock obtained the guns. Chris Sullivan, owner of the Guns & Guitars shop near Paddock's home in Mesquite, Nevada, confirmed Paddock had legally purchased firearms from the store but did not offer more detail.

Two officials familiar with the investigation told the AP that Paddock had two bump stocks. They were investigating whether those stocks were used to modify weapons used in the massacre.

The website for one such product, Slide Fire, shows several videos in which shooters launch multiple rounds per second in bursts that sound almost indistinguishable from automatic fire. Reviewers have reported that the product permits a shooter to go through hundreds of rounds every minute.

The company did not return requests for comment.

Paddock could also have illegally converted semi-automatic rifles into fully automatic guns, a process made easier by the presence of guides uploaded to the internet.