WASHINGTON — It has been six weeks since Las Vegas shooter Stephen Paddock attached a device to his semiautomatic firearm that effectively turned his rifle into a battlefield machine gun, leaving 58 people dead and hundreds more injured in a matter of minutes.

Within days, House Speaker Paul D. Ryan, R-Wis., and other Republican lawmakers – including some of Congress’s most avid supporters of gun rights – said they would consider restrictions on the accessory, known as a “bump stock.”

Even the National Rifle Association unexpectedly joined the effort to restrict bump stocks, molded pieces of plastic or metal that, when attached, allow a gun to fire up to 100 rounds in seven seconds.

“The NRA believes that devices designed to allow semi-automatic rifles to function like fully-automatic rifles should be subject to additional regulations,” the NRA said in a statement shortly after the shooting.

But no action has been taken by Congress. Instead, Republican lawmakers punted the responsibility to the Bureau of Alcohol, Tobacco, Firearms and Explosives, which in turn says it cannot regulate bump stocks unless Congress changes the law.

A bump stock is attached to a semi-automatic rifle at the Gun Vault store and shooting range in South Jordan, Utah.

The only thing that has happened with bump stocks since the Las Vegas shooting is that a leading manufacturer of the device, Slide Fire, resumed selling them. Slide Fire had announced in the wake of the shooting that it was temporarily suspending sales.

Several retailers, including Walmart, removed bump stocks from their shelves. But other gun dealers reported that sales of bump stocks spiked after Las Vegas and some stores sold out of the devices quickly.

“We would like to take the time to thank all of our customers for their patience and support throughout this past month,” Slide Fire said in an email when the company began selling bump stocks again Nov. 1. The company did not respond to requests for comment.

Four days after the Las Vegas shooting, Ryan, House Majority Leader Kevin McCarthy, R-Calif., and House Judiciary Committee Chairman Bob Goodlatte, R-Va., said they would consider restrictions on bump stocks.

“Fully automatic weapons have been outlawed for many, many years,” Ryan said. “This seems to be a way of going around that, so obviously we need to look how we can tighten up the compliance with this law so that fully automatic weapons are banned.”

But the next week, on Oct. 11, Ryan backed away from his comments about legislative action. Instead, he and many other House Republicans said they hoped ATF would act administratively to outlaw the devices.

“We think the regulatory fix is the smartest, quickest fix, and then, frankly, we’d like to know how it happened in the first place,” Ryan told reporters.

Nine senators wrote ATF Acting Director Thomas E. Brandon asking that ATF “review the Obama administration’s interpretation and issue your own interpretation.”

In 2010, ATF said that the agency did not need to regulate bump stocks because they do not fit the legal definition of a machine gun.

A 1986 law bans the sale of machine guns manufactured after 1986 and restricts the sale of machine guns made before then. But bump stocks do not permanently alter a gun’s trigger mechanism so ATF says it does not fall under the law. The companies selling the device configured the bump stock so it harnesses a gun’s natural recoil, allowing it to bounce back and forth off a shooter’s trigger finger.

“No permanent modifications,” Slide Fire advertises in its promotional video.

NRA spokeswoman Jennifer Baker said she had no further comment beyond the organization’s original statement six weeks ago that the devices “should be subject to additional regulations.”

When asked what ATF is doing in response to Congress’s concern about bump stocks, a spokeswoman said she could not comment.

“ATF does not comment on internal deliberations,” said ATF spokeswoman Mary E. Markos. “The Department of Justice is reviewing its regulatory authority in this space. House and Senate Judiciary Committee staffers were recently briefed by ATF regarding bump stock related matters.”

But Sen. Dianne Feinstein, D-Calif., the ranking Democrat on the Senate Judiciary Committee, said that at a committee briefing on Oct. 12, ATF officials told senators that it has concluded it lacks the authority to ban bump stocks.

In addition, current and former ATF agents wrote a letter to lawmakers, saying that it is Congress that must take action. Under the law, they wrote, attaching a bump stock to a gun “does not make it a machine gun,” so ATF cannot regulate it.

“The law is very clear, and it does not currently allow ATF to regulate such accessories,” Michael R. Bouchard, former ATF assistant director and now president of the ATF Association, wrote in the letter, which was obtained by The Washington Post.

Feinstein, who has long advocated for stricter gun control laws, has introduced a bill that would ban the sale and possession of bump-stock equipment and other devices that essentially turn a semiautomatic weapon into an automatic one.

“I continue to believe that legislation to ban bump-fire stocks is needed to save lives,” Feinstein said. “Machine guns have been banned for more than 30 years, so banning accessories that allow semi-automatic weapons to achieve a fully automatic rate of fire should be a no-brainer.”

She is still trying to find a Republican co-sponsor.