Congress learns what a bump stock is

But after the shooting in Las Vegas, Sen. Jeff Flake, a Republican from Arizona, was stunned by the audio he was hearing, the sounds of a gun popping at a concert in Las Vegas at a frequency he was almost sure wasn't just coming from a semi-automatic weapon.

"I think all of us assumed that when we heard the audio that it was a fully automatic weapon. ... How in the world would the shooter get that?" Flake asked.

Flake called a friend who collected guns and asked him: What was he hearing?

"He heard the audio and he thought 'that wasn't fully automatic. Fully automatic is about 800 rounds, and that sounded like about 500.' ... He started telling me about, not just the bump stocks, about a crank and other methods to mechanically increase the rate of fire," Flake said.

Sen. John Cornyn, the majority whip who called for hearings on the bump stocks this week, also said it was the video footage that got him thinking about the gun accessory.

"You know, I'd heard about them before, but I'd never actually seen one, and I certainly had never seen a video of someone using them to spray a concert audience of 22,000 people with gunshots," Cornyn said. "I just had no idea that that was possible."

Across Capitol Hill, the contours of America's gun debate are well established. In the wake of mass shootings, Republicans retreat to their positions and Democrats to theirs. But bump stocks -- devices that weren't part of the American lexicon before Monday -- have shed new light on an old debate.

"I think we ought to have hearings on it, look at it, and I think we shouldn't be marketing or allowed to market things that make our guns illegal like they do," said Sen. Shelley Moore Capito, a West Virginia Republican.

It wasn't just Republicans learning about the device this week. Sen. Tim Kaine, a Democrat who was governor during the Virginia Tech shooting in 2007, said that it was his son, a member of the Marine Corps, who got him up to speed on the accessories that convert semi-automatic weapons to firing more like an automatic one.

"Within five hours of the shooting, he had written me a note sort of walking me through how you do it and how simple it is," Kaine said.

What makes the bump stock debate different, members say, is the fact that automatic weapons -- guns that unleash bullets in rapid succession with just a single pull of the trigger -- are already highly restricted in the US. Therefore, members argue, it's hard not to take a look at a device that would transition a legal weapon into one that fires at a rate similar to the tightly restricted ones.

House Speaker Paul Ryan -- a hunter -- said Thursday on MSNBC that he wanted to look more into bump stocks given the laws that are already on the books.

"I didn't even know what they were until this week, and I'm an avid sportsman," Ryan said. "So, I think we're quickly coming up to speed with what this is. Fully automatic weapons have been banned for a long time. Apparently this allows you to take a semi-automatic, turn it into a fully automatic. So clearly that's something we need to look into."

In the House, Rep. Carlos Curbelo, has taken the lead on introducing legislation to ban the sale of bump stocks. The Florida Republican told reporters Thursday that so far many Republican members have reached out to him interested in signing onto his bill.

"I think we are on the verge of a breakthrough when it comes to sensible gun policy," Curbelo told reporters Thursday, saying his office has been "flooded" with calls from other lawmakers asking about the bill.

It's still early in the debate over bump stocks. Even Cornyn, the Republican from Texas calling for a hearing, hasn't said he wants to ban bump stocks with legislation. The Senate's Judiciary Chairman Chuck Grassley has said he'd need a full investigation into what happened in Las Vegas before holding any hearing. And while the NRA signaled an openness to some changes on bump stock regulations, the group's statement called on the Bureau of Alcohol, Tobacco, Firearms and Explosives to make the changes, not Congress.

In the meantime, many GOP senators remain cautious about throwing their support behind regulating an accessory they are just learning about.

"I am going to get someone to present to me in my office what a bump stock device is. I apologize but I do not know," Corker said earlier this week. "We have a person in our office who sits down and walks me through all these technical issues and shows me photographs and tells me how that affects law. So I don't have a response today."