Two weeks ago, bump stocks were just an odd-sounding firearm attachment largely unknown outside gun enthusiast circles.

That all changed early last week with the deadly shooting in Las Vegas where police discovered a dozen of the devices in the shooter's hotel room overlooking the city's neon-lit Strip. Now, Republicans and Democrats in Congress, the National Rifle Association and other pro-gun groups are asking for a fresh look at the legality of bump stocks.

"We think the regulatory fix is the smartest, quickest fix," House Speaker Paul Ryan, R-Wis., said Wednesday during his weekly news conference. "We are still trying to assess why ATF let this go through in the first place."

The ATF is the Bureau of Alcohol, Tobacco, Firearms and Explosives, which is responsible for making a classification decision on firearms, their parts and accessories. The ATF declined to comment on any pending or potential future review of bump stocks.

But the ATF did finish a classification review of a bump stock, also known as a slide fire, in January 2010. It concluded that the device was a firearm part, not a machine gun, and therefore it was not regulated under the Gun Control Act or the National Firearms Act.

Machine guns, which are fully automatic, are strictly controlled in the United States. It is illegal for a private citizen to own a machine gun manufactured after 1986, although one can own, sell or purchase an automatic weapon made before that date.

Under U.S. law, a machine gun is defined as "any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger."

It is that last phrase — "by a single function of the trigger" — that is key to understanding ATF decisions, said Rick Vasquez, a firearm consultant and former acting chief of the ATF's firearms technology branch, which conducts classification reviews.

If a gun fires more than one bullet with a single pull of the trigger, then by law it is considered a machine gun. If, however, a gun fires only one bullet for each pull of the trigger, it is not.

Bump stocks, Vasquez said, enable an accelerated shooting rate, but the way they are designed ensures that each pull of the trigger only unleashes one bullet. That means, he said, that they don't qualify as machine guns under current law.

And while the NRA and some lawmakers are calling on the ATF to review the classification of bump stocks, former ATF officials and gun control advocates say that's a difficult hill to climb for several reasons.

For one, the bureau's hands are tied by current law, said Kristen Rand, the legislative director of the Violence Policy Center, which advocates for gun control.

"The ATF can't do anything about bump stocks without a change to the statute," she said. "Basically, all they can determine is whether or not a device causes a gun to fire in fully automatic mode, and it's banned, or it does not, in which case it can't be regulated."

But the law hasn't changed since the ATF originally signed off on bump stocks, so a regulatory change would require the bureau to reverse its original decision on the devices without Congress writing a new law or changing the controlling laws.

"If I looked at it again, looked at the same documents that we had at that time, then my opinion wouldn't change," Vasquez said.

David Chipman, a 25-year special agent with ATF who now serves as a senior policy adviser at the gun control group Americans for Responsible Solutions, agreed.

"If you ask me today if bump stocks meet the definition of full automatic, I would say no," Chipman said. "The trigger is functioning, so that's why my sense is that throwing it back to ATF can't possibly be the solution."

ATF could, in theory, change its mind under political pressure, Chipman said, but that would open it up to legal challenges from bump stock owners and manufacturers.

"If ATF just summarily chooses to make someone's product highly regulated after saying that it's not, it's likely to go to court," he said. "The only fix I think that would be quick would be for legislators to pass a new law."

There has been movement on Capitol Hill, where some lawmakers are pushing for a change to the law.

In the Senate, California Democrat Dianne Feinstein has proposed a bill that would make it illegal to buy, sell, manufacture or possess a device or accessory that is designed to accelerate the rate of fire of a semi-automatic rifle. That would cover bump stocks.

On the House side, Florida Republican Carlos Curbelo has proposed a similar bill.

Curbelo told NPR last week that the response from House Republicans to his proposal has been "overwhelming."

"I'm very confident that today a majority of Republicans in the House would support banning these devices," he said.

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