A bump stock — a device that lets a semi-automatic weapon fire off shots rapidly, like the kind found in the hotel suite Las Vegas shooter Stephen Paddock used as a sniper's nest Sunday night — is perfectly legal to buy in New Jersey.

But there's a catch. Connecting one to an AR-15-style gun will change the weapon's classification — turning it into an illegal assault weapon under New Jersey law, attorney Evan F. Nappen said.

An AR-15-style weapon can legally have one “offending feature” — for most such guns, that's the pistol grip, he said. But “if you have two or more offending features on an AR then it is an assault firearm and prohibited.”

He noted because the bump fire stock utilizes a pistol grip and a telescoping stock “that would make it two offending features, and turn whatever semi-automatic firearm you would attach it to into a prohibited assault firearm, per the Attorney General’s guideline.”

Nappen said even if you simply buy a bump stock and own a semi-automatic rifle, you are technically in violation of the law.

“If you have the part or combination of parts that make something an assault firearm it does not necessarily require assembly,” he said.

That can mean up to 10 years in state prison, with a mandatory minimum 3.5-year sentence. And such a felon would lose gun rights nationwide.

Nappen — an advocate for less-strict gun laws and a board Member of the Association of New Jersey Rifle and Pistol Clubs — said several of his clients have been charged with possessing assault firearms because of modifications made to AR-15-style rifles. But he couldn't recall a case with a bump stock.

Joe Hawk, the owner of Guns and Roses gun shop in Toms River, said he doesn’t sell bump stocks in his store — and believes they shouldn't be sold at all.

“There’s no reason for it. There’s no need for it. It’s only for someone who’s trying to increase their rate of fire," he said. "The only reason you would use something like an increased rate of fire would be in a war zone.

“The only reason that you’d want to do that is to put yourself on a battlefield, and American streets and American communities are not a battlefield."

Though legally and widely available, the so-called "bump stocks" have attracted scrutiny from authorities and lawmakers in recent years.

California Sen. Dianne Feinstein, who has long expressed concern over the availability of such accessories, said Tuesday Paddock modified at least one of his weapons with a bump stock device, but she did not elaborate. Law enforcement officials speaking on the condition of anonymity have said the same.

"Individuals are able to purchase bump fire stocks for less than $200 and easily convert a semi-automatic weapon into a firearm that can shoot between 400 and 800 rounds per minute and inflict absolute carnage," she said, calling for a ban on their sale.

A semi-automatic weapon requires one trigger pull for each round fired. With a fully automatic firearm, one trigger pull can unleash continuous rounds until the magazine is empty. The bump-stock devices work by manipulating the trigger mechanism extremely rapidly, far faster than a person could do so without them.

Authorities found two bump stocks in Paddock's hotel room, two officials familiar with the investigation told The Associated Press. They are investigating whether those items were used to modify weapons used in the massacre, according to the officials, who were briefed by law enforcement and spoke on condition of anonymity because the investigation is still unfolding. Sheriff Joseph Lombardo hasn't said whether they played a role in the rampage.

Authorities say Paddock opened fire from the windows of his 32nd floor hotel room late Sunday, killing 59 people and wounding hundreds more at a country music festival. Police stormed his room and found he had killed himself after committing the deadliest mass shooting in modern U.S. history.

Witnesses and law enforcement official said the quick, 50-round bursts of gunfire raised the possibility that Paddock had used a fully automatic weapon or modified his semi-automatic rifles to function like one. Paddock had 23 guns in his hotel room.

Hawk, the New Jersey gun retailer, said he’s never heard of a hunter buying a bump stock.

“I don’t believe it should be legal. It’s letting someone getting very close to a military type of firearms and that belongs on a battlefield," he said.

Jay Ridgeway, who works at the gun counter at Sportsmen’s Center in Bordentown, said his store does not sell bump stocks either.

“I’m shocked that it’s legal anywhere because you can’t have fully automatics, so if you have something that makes your gun fully automatic, that would be silly,” he said.

Ridgeway pointed out he’s never even seen a bump stock in a gun shop and “I don’t know why anybody would carry it.”

Nappen wondered out loud if simply banning bump stocks outright would make any difference and prevent the kind of tragedy we’ve had in Vegas from taking place.

“Do you really think that banning anything is going to stop somebody from committing mass murder? We see it done with trucks, we see it done with bombs, we see it done with all kinds of things, legal and illegal,” he said.

He added if the man behind the Las Vegas rampage did not have multiple guns, he could have tossed hand grenades off of his balcony and probably killed even more people than he did.

A call to the Attorney General’s office was not immediately answered.

An example of a bump stock, and how it affects a gun's operation, can be seen here:

— With additional reporting by the Associated Press

Contact reporter David Matthau at David.Matthau@townsquaremedia.com