Bump stocks: Congress considering legislation to ban the gun attachment Las Vegas shooter used

— -- Bump stocks were virtually unheard of on Capitol Hill until this week. Even some anti-gun lobbyists told ABC New this week that the gun attachments weren’t on their radar before the massacre in Las Vegas Sunday.

But that that is all changing, and fast.

A weapons expert described the so-called bump stock as a legal attachment that simulates automatic fire on semiautomatic weapons. In essence, it uses the momentum of the gun's recoil to rapidly bump the trigger, allowing the gun to fire much faster, albeit with less accuracy.

The expert, who requested anonymity, said a shooter could fire 550 to 650 rounds a minute with a bump stock attachment on an AR-15 or AK-47. The Las Vegas shooter used the device.

Sen. Dianne Feinstein, D-Calif., a longtime advocate for gun control, introduced legislation Wednesday to ban bump stocks. She authored legislation to ban assault weapons in 2013 that failed in the Senate, 40-60. Democrats will need significant Republican support for anything to pass.

More than a dozen influential Republican lawmakers have told ABC News they are either supportive of a ban, or at least favor reviewing the issue.

“The fact that fully-automatic weapons are already illegal and this makes another weapon capable [of automatic fire]; I would be supportive of that,” Senate Homeland Security Chairman Ron Johnson, R-Wis., said when asked Wednesday about legislation to ban bump stocks.

Senate Minority Whip John Cornyn, a Texas Republican, told reporters Wednesday there should be a hearing.

"I talked to Chairman [Chuck] Grassley of the Senate Judiciary Committee, and I believe that once the investigation is complete and we learn all aspects of what contributed to this event, then we should have a hearing and look into it," Cornyn said, referring to the Iowa Republican.

Meanwhile, similar legislation to regulate bump stocks in emerging in the House of Representatives. Florida Republican Rep. Carlos Curbelo said he would introduce a bipartisan bill as soon as today that would call for a total ban.

“The goal is to prohibit these deadly devices that caused so much death and destruction in Las Vegas earlier this week,” Curbelo told ABC News in an interview. “Most members on both sides of the aisle agree that this is a blatant circumvention of the law and we want to close this loophole.”

Federal law prohibits manufacturers from producing automatic weapons for civilian use.

The National Rifle Association (NRA), the powerful pro-gun lobby that threw $21 million behind Donald Trump’s presidential candidacy last year, weighed in on the debate for the first time today, calling on the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) to review the matter, rather than create new laws.

Banning guns, the NRA said in its statement, will do "nothing to prevent future attacks."

The ATF approved the sale of bump stock attachments on June 20, 2010, and again in 2012, ruling the device complies with federal law because it has "no automatically functioning mechanical parts."

Some Republicans are asking why these bump stock parts were ever approved in the first place and are calling out former President Obama.

"We need to determine what happened in 2010 when the Obama administration approved those - the why, the what, what was the process they were going through to do the approval,” Sen. Jim Lankford, R-Okla., told reporters Wednesday on Capitol Hill.

Others are asking the ATF to revisit that decision.

"I’d like to hear from the ATF why do you believe it doesn’t violate the spirit of the law?" Sen. Lindsey Graham, R-S.C., told reporters.

Several House Republicans, including Rep. Adam Kinzinger, R-Ill., are planning to send a letter to the ATF asking for a review of previous decisions regarding the legality of bump stocks and similar accessories.

"I think the ATF can do this quickly; they can make a determination. If they drag their heels on it, as we say in the letter, we’ll look at legislation solutions," Kinzinger said.

ABC News' Ali Rogin, MaryAlice Parks, John Parkinson and Benjamin Siegel contributed to this report.