“The N.R.A. believes that devices designed to allow semiautomatic rifles to function like fully-automatic rifles should be subject to additional regulations,” the group said.
Last year, the N.R.A.’s online magazine, America’s First Freedom, called one of the rapid-fire devices “sublime,” and advised users to keep copies of the firearms bureau’s ruling that such items are legal.
Sales of bump stocks surged this week, after news emerged that they might have been used in the Las Vegas shooting. Some retailers ran out of the devices, even as others decided to stop offering them on their websites.
Democrats have supported legislation banning such mechanisms, particularly a bill introduced in 2013 by Senator Dianne Feinstein of California, while Republicans generally have not. But since the massacre, several leading Republicans, including Senator John Cornyn, the No. 2 Republican in the Senate, have raised serious questions about the devices.
In the House, Representative Carlos Curbelo, Republican of Florida, has drafted a measure banning bump stocks, and he said on Thursday that his office has been flooded with calls from “dozens” of his fellow Republicans, who want to sign on.
“I think we are on the verge of a breakthrough when it comes to sensible gun policy,” Mr. Curbelo said.
His comments followed those of House Speaker Paul D. Ryan, who during a morning interview with MSNBC also raised questions about the conversions, and said he was open to legislation. “Clearly that’s something we need to look into,” Mr. Ryan said.
Separately, Representative Adam Kinzinger, Republican of Illinois, is circulating a letter among his colleagues, calling on the A.T.F. to re-evaluate bump stocks, which he said have “no place in civilized society.”
“The A.T.F. must re-evaluate these devices,” Mr. Kinzinger wrote, “and it is my hope that they conclude these mechanisms violate the spirit, if not the letter, of the law.”
But gun control advocates drew a sharp distinction between a ban, like that proposed by Senator Feinstein, and merely asking the firearms agency to re-evaluate the matter, as the N.R.A. and Mr. Kinzinger have.
“If they had actually said we support bipartisan legislation, you might have characterized it as a concession,” said John Feinblatt, president of Everytown for Gun Safety. “But they’re actually just punting it back to the very federal agency that said bump stocks were legal. I don’t think this is a concession, it’s just a wink and a nod.”
In its statement, the N.R.A. also urged Congress to pass one of the group’s highest legislative priorities, a bill that would allow anyone who is licensed by one state to carry a concealed weapon to do so in any other state that allows concealed carry. The organization did not explicitly tie concealed carry reciprocity to regulation of rapid-fire devices, but another pro-gun group did.
Gun Owners of America said Thursday that it opposed a bump stock ban, but added in a statement, “If law makers want a vote on bump stocks, they should vote on reciprocity as well.”
Senator Richard Blumenthal, a Connecticut Democrat, accused the N.R.A. of pursuing “a road to nonaction.” He noted that a regulation — as opposed to a law — could take years to enact, and that Democrats have opposed the reciprocity measure.
“I think it’s a highly dangerous and deceptive dodge,” Mr. Blumenthal, who is co-sponsoring legislation to ban bump stocks, said in an interview. “It’s typical N.R.A. evasion and avoidance of reality; the simple, stark political fact is there is growing support for a ban, a complete prohibition.”
On a fully automatic weapon, when the user pulls the trigger, the weapon will quickly fire round after round, until the trigger is released or the magazine empties. On a semiautomatic, each pull of the trigger fires a single bullet, and reloads the weapon for the next shot. Military assault rifles can be fired either in automatic or semiautomatic mode; similar-looking guns sold on the American civilian market are semiautomatic only.
But there are a few devices on the market that make a semiautomatic weapon mimic the much higher rate of fire of a fully automatic one — though accuracy typically suffers, and the weapon can overheat, damaging it or quickly making it hard to handle. The devices include bump stocks, trigger cranks, and what are sometimes called rapid-reset triggers, any of which allow a semiautomatic weapon to fire faster than a human finger can pull a trigger, unassisted.
Recordings of the Las Vegas shooting include the sound of extremely rapid shooting, and police officers responding to the scene radioed that they believed they were under fire from fully automatic weapons. That could be explained by the bump stocks Mr. Paddock had, though law enforcement officials have not said definitively whether he actually used those devices.
The officials have said Mr. Paddock, who fatally shot himself, fired multiple rifles during the shooting, which would be consistent with weapons fitted with bump stocks overheating. Photographs that have leaked of the hotel suite appear to show his left hand encased in a mitt, which could have been protection against overheating.