Pro-gun lawmakers in the U.S. admit to being clueless until recently about bump stocks, devices that enabled a Las Vegas gunman to speed up the rate of his gunfire, and spray hundreds of rounds that killed at least 58 people and wounded hundreds.

"I didn't even know what they were until this week," said Paul Ryan, the Republican House leader and an avid hunter.

"This is such a new component to me," said Jeff Duncan, a South Carolina congressman vocal about expanding gun rights.

"I own probably more guns than most members of Congress, and that was the first time I'd ever heard of a bump stock," said North Carolina Congressman Mark Meadows, chair of the ultra-conservative Freedom Caucus.

Rare bipartisan consensus

This general unawareness that bump stocks even exist might help explain the rare bipartisan consensus on gun reform this week.

Republican leaders in the House and Senate are signalling a willingness to back a bill to outlaw bump stocks. The device was used by Las Vegas gunman Stephen Paddock on Sunday to modify a semi-automatic into a weapon that could legally mimic the rapid firing of highly restricted fully automatic weapons.

LASVEGAS-SHOOTING/GUNS

Republicans signalled Thursday that they would be willing to consider outlawing bump stocks, left, components that attach to a semi-automatic rifle to increase its firing rate. Twelve of the semi-automatic rifles that Stephen Paddock had with him during Sunday's mass shooting were fitted with bump stocks. (George Frey/Reuters)

The National Rifle Association, normally quiet in the immediate aftermath of mass shootings, released a statement on Thursday calling for more regulation of bump stocks.

Why the powerful gun lobby would side with gun-control advocates on bump stocks comes down to how narrow the bill appears to be, says former NRA lobbyist Richard Feldman.

'Keep my powder dry'

"I think they know not to get terribly exercised over this," said Feldman. "I keep my powder dry for the important fights. There's no fear there on my part."

No fear, maybe, because Feldman had no knowledge of the existence of bump stocks until now anyway.

Given that there were no breakthroughs on gun legislation after other recent mass shootings, including the killings of 20 children and six teachers at Sandy Hook Elementary School in 2012, conservative support for a ban on bump stocks has buoyed some liberals.

"There is momentum, and a ban on bump stocks has a good chance," said David DeGrazia, a George Washington University ethics professor who has argued for moderate gun control and is co-author of the book Debating Gun Control​.

DeGrazia wrote in an email to CBC News that prior to the recent show of bipartisan support for limitations on bump stocks, he was convinced that "even the most sensible" new gun restrictions would fail to materialize despite the carnage in Las Vegas.

Viewed as 'novelty items'

What may have made this an easier call for the gun lobby and pro-gun conservatives, though, is market obscurity.

In a 2010 letter to the Bureau of Alcohol, Tobacco, Firearms and Explosives, the bump-stock manufacturer Slide Trigger described their product as "intended to assist persons whose hands have limited mobility" fire rifles more rapidly.

Sociologist Jennifer Carlson, who studies gun culture at the University of Arizona, said the devices are not widely possessed by gun owners and are viewed as "novelty items." 

Congress Taxes Ryan

'I didn't even know what they were until this week,' said Paul Ryan, Republican House leader, about bump stocks. Ryan is an avid hunter and an opponent of gun control legislation. (Patrick Semansky/Associated Press)

The sliding bump stock attaches to a rifle and uses the recoil to bounce the gun against a person's finger on the trigger. The accelerated interval between rounds can be as fast as a fully automatic weapon, though there's some debate among gun experts about whether a gun equipped with a bump stock actually becomes a more lethal weapon.

The NRA's public call for review of regulations for bump stocks reminds Carlson of how the association handled calls for background checks. The NRA supported expanded background checks in the 1990s, but objected to the actual proposed details of the legislation.

"The real test is what happens when legislation is on the table."

Broader ban would meet resistance

In this case, she said, the NRA "could say bump stocks should be illegal, but that all the ways of making this illegal will have the unintended consequences of making semi-automatics illegal."

She predicts a narrow legislative proposal could have wide support, while a broader ban would meet resistance from an NRA that has often taken a "no compromises" approach to gun reforms, arguing that restrictions could be a slippery slope to dismantling the Second Amendment.

"I've got to confess, I really don't know what bump stocks do," said Robert Cottrol, a law professor and staunch Second Amendment supporter at George Washington University.

"But one of the difficulties we often have in terms of getting compromise in this area is that people who advocate different measures themselves frankly admit it's just the first step towards something stricter."

Rate-of-fire ban

If Democrats push for legislation cracking down on general rates of fire, it could be exceedingly difficult to enforce, given all various hacks and devices on the market.

(A "bump firing" technique can be practised, with the shooter interacting with an AR pistol on the hip to use kickback in a similar way to speed up a trigger pull without any apparatus. Gun hackers have also been known to publish tutorials for using rubber bands to convert semi-automatics into something closer to a fully automatic gun.)

Las Vegas Shooting Victims

Some of the 58 victims of the mass shooting at a country music festival in Las Vegas on Sunday. Nearly 500 people were also wounded. (The Associated Press)

The NRA might argue that rate-of-fire laws would put semi-automatics in jeopardy of legal regulation.

For now, at least, bipartisan support on a bill restricting bump stocks allows Republicans and Democrats to say they passed something — anything — in response to the massacre in Vegas, the deadliest mass shooting in modern U.S. history.

The Gun Owners of America, a national organization with 1.5 million members, published a statement on Thursday saying it opposed any ban on bump stocks because they "help gun owners with disabilities fire their weapons."

Nelson Lund, a Second Amendment expert at George Mason University, was unaware of the device until recently. On Monday, he said the only way he could foresee any new federal gun restrictions resulting from the Vegas shooting would be if someone could point to a particular measure that would have prevented the scope of that tragedy.

"I told you before I didn't think anything was likely to happen," he said Thursday. "Since that time, I've learned something I didn't know."

USA-CONGRESS/REPUBLICANS

'I own probably more guns than most members of Congress, and that was the first time I'd ever heard of a bump stock,' said Republican Congressman Mark Meadows on Thursday. (Joshua Roberts/Reuters)