How does the device the Las Vegas gunman used to create a fully automatic rifle work? A Utah gun lobbyist shows us.

South Jordan • When Clark Aposhian first watched snippets of Sunday’s massacre in Las Vegas, he knew the gunman hadn’t used a machine gun or similar fully automatic weapon.

To the untrained ear, the shots sounded like the rapid stoccato of a machine gun, which fires more than one round with each trigger pull. But Aposhian, chairman of the Utah Shooting Sports Council, knew better. He’s one of the few Americans who legally owns fully automatic guns, and the sound of the gunman’s shots was something different — slower and more irregular.

“I said, ’I bet you anything we find out it’s a bump-fire-type device,’” Aposhian recalled telling a friend.

He was right.

Police say they recovered 23 firearms in Stephen Paddock’s 32nd-floor suite at the Mandalay Bay Resort and Casino. And at least a dozen had been legally modified to fire like automatic weapons, utilizing an attachment known as a bump-fire stock.

The devices — which have existed for less than a decade — allow a semiautomatic rifle to transform into a fully automatic weapon and drain a large magazine in seconds.

Aposhian has a bump stock for his AK-47 assault rifle, one of hundreds of firearms he owns, and he showed it to a Salt Lake Tribune reporter at The Gun Vault shooting range Wednesday. It is a moveable plastic piece that rests against the shooter’s shoulder and attaches to the back of a gun’s receiver.

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 then causes the gun to buck back and forth, repeatedly “bumping” the trigger against the finger. The bump stock allows for this movement.

Technically, that means the finger is pulling the trigger for each round fired, maintaining the legal characteristics of a semiautomatic weapon. The rapid fire does not necessarily make the weapon more lethal — much of that would be dependent on the type of ammunition used. But it does allow the person firing the weapon to fire shots more quickly.

The government approved bump-stock sales in 2010 after concluding that they did not violate federal law. But Aposhian said a technique for firing a semiautomatic weapon in an automatic fashion has been around since semiautomatic guns were invented. It involves shooting the rifle from the hip, where it can buck back and forth freely, without running into the shooter’s shoulder.

“The stock is nothing more than a novelty to accentuate [this] firing technique,” Aposhian said.

Machine guns still shooter faster and more consistently than a gun with a bump stock— about 750 to 950 rounds per minute versus “at best” 550 rounds for a bump-stock-equipped rifle, Aposhian said. And the fact that bump stocks are unreliable and inaccurate probably didn’t matter to Paddock.

“You just can’t get accurate with this stock. You cannot,” Aposhian said. “But for what this looney did down in Vegas? You don’t have to be that accurate to hit a crowd of people.”

Aposhian isn’t a fan of bump stocks. Neither were two Gun Vault employees who tried Aposhian’s AK-47 with the device Wednesday, describing the shooting sensation as “weird” and “awful.” “It’s so dang hard to control,” Aposhian added.

(Rick Egan | Tribune File Photo) Clark Aposhian gives instructions during a free concealed firearms permit class to Utah educators at the Salt Lake City Library, Friday, January 3, 2014.

So why did Aposhian buy one?

“I’m a gun lobbyist,” he said. “I’ve got all the new guns, I’ve got all the old ones. So it was time [to buy a bump stock], and I got one. I quickly realized that my ammo’s expensive, and this is a great way to waste ammo.”

The original idea behind the bump stock was to make it easier for people with disabilities to shoot a gun. But Clark suspects someone came up with the design first, then later “came up with a reason for it.”

It’s unclear how many have been sold. They can be found online for less than $200. The industry leader, Slide Force, has a Facebook page filled with videos extolling its features. In one video, a woman says, “It’s so easy because once you slid it forward and leaned into it, it just fires.” In another video, a man fires 58 rounds to celebrate his 58th birthday — in 12 seconds.

But bump stocks aren’t useful for hunting, or even self-defense, Aposhian said, “unless you were defending yourself against a horde of people.”

They are, however, a way for people to go to the shooting range and feel like they’re shooting a real machine gun. Fully automatic firearms were banned in 1986; only those manufactured before that year are legal, and many now go for tens of thousands of dollars.

“It’s a recreational item,” Aposhian said of bump stocks. People buy them “because it’s fun.”

The Associated Press contributed to this story.