Mr. Cornyn said the continuing legality of the conversion kits was “a legitimate question,” and told reporters he had asked Senator Charles E. Grassley of Iowa, the Judiciary Committee chairman, to convene a hearing on that issue and any others that arise out of the Las Vegas investigation.
Other Republican senators, including Lindsey Graham of South Carolina, Orrin G. Hatch of Utah and Marco Rubio of Florida, said they would be open to considering legislation on bump stocks.
“We certainly want to learn more details on what occurred in Las Vegas,” Mr. Rubio said, “and if there are vulnerabilities in federal law that we should be addressing to prevent such attacks in the future, we would always be open to that.”
In the House, Representative Carlos Curbelo, Republican of Florida, said he was drafting bipartisan legislation banning the conversion kits. Representative Mark Meadows, the head of the conservative Freedom Caucus, also said he would be open to considering a bill, while Representative Bill Flores, Republican of Texas, called for an outright ban.
“I think they should be banned,” Mr. Flores told the newspaper The Hill. “There’s no reason for a typical gun owner to own anything that converts a semiautomatic to something that behaves like an automatic.”
In an often deadlocked Washington, none of the pronouncements guaranteed action. The National Rifle Association, which has poured tens of millions of dollars into Republican campaign coffers, remained mum on the bump stock discussion and could stop it cold.
And Erich Pratt, executive director of another gun rights group, Gun Owners of America, vowed to block any legislation.
“We see this as an item that is certainly protected by the Second Amendment, and realistically, they are already on the market, so passing a law banning them isn’t going to stop bad guys like this creep in Las Vegas,” he said.
But Senator Dianne Feinstein, the California Democrat, tried to force the issue, introducing legislation, backed by about two dozen Democrats, that would ban bump stocks.
Ms. Feinstein cautioned that bipartisan support for such narrow legislation would hardly constitute a sea change. She tried to ban bump stocks in 2013, but that was part of broader legislation to renew the assault weapons ban, which went nowhere.
“I mean, if not this, what?” she asked. “It doesn’t take a weapon away. It just means you can’t convert it into something it’s not meant to be.”
At a hastily convened news conference, Ms. Feinstein said the Las Vegas massacre, which left 58 people dead and about 500 injured at a country music festival Sunday night, had hit home with her. Her daughter had planned to attend the concert but decided against going at the last minute.
Ms. Feinstein, who has spent years shepherding gun safety legislation — almost always unsuccessfully — said she introduced the measure on the advice of Senator Chuck Schumer of New York, the Democratic leader, who reasoned that by offering a narrowly tailored provision, she might get Republican support.
“He called and thought that this would be something that’s small, in isolation, that might get a broad coverage of people,” Ms. Feinstein said.
Bump stocks replace a rifle’s standard stock, which is the part held against the shoulder, freeing the weapon to slide back and forth rapidly, harnessing the energy from the kickback that shooters feel when the weapon fires. The stock “bumps” back and forth between the shooter’s shoulder and trigger finger, causing the rifle to rapidly fire again and again, far faster than an unaided finger can pull a trigger.
In marketing the devices, two Texas companies, Bump Fire Systems and Slide Fire Solutions, were apparently concerned that they would not be legal. But in June 2010, after an inquiry from Slide Fire, the Bureau of Alcohol, Tobacco, Firearms and Explosives, or A.T.F., sent a letter saying that the company’s bump stock product “is a firearm part and is not regulated as a firearm under the Gun Control Act or the National Firearms Act.”
The letter, which Slide Fire posted on its website, noted that the stock “has no automatically functioning mechanical parts or springs and performs no automatic mechanical function when installed.”
It said that the device was “intended to assist persons whose hands have limited mobility.”
Bump Fire posted a similar letter from April 2012 to its website.
The Las Vegas gunman fired down on the concertgoers from the 32nd floor of a nearby hotel, the Mandalay Bay. With his fixed firing positions and distance from his victims, he almost certainly was more lethal because of the conversion kits. But until the shooting, many lawmakers said, they had never heard of bump stocks.
The devices were introduced during the past decade by Bump Fire and Slide Fire, both based in Moran, Tex., a town with fewer than 300 residents near Abilene. Bump Fire’s website appeared to be down for much of Wednesday. The company wrote on its Facebook page on Tuesday that its servers had been overwhelmed by “high traffic volume.”
Multiple items on Slide Fire’s site on Wednesday featured the notice, “Due to extreme high demands, we are currently out of stock.”
Bump Fire sells stocks for an AK-47 and an AR-15 for $99.99 each. Slide Fire’s stocks are priced between $140 and $300. Neither company responded to a request for comment.
But the Las Vegas massacre has apparently been good for the bump stock business; on Gunbroker.com, an auction site for firearms and shooting accessories, at least three dozen listings featuring bump stocks had attracted multiple bids.
Zack Cernok, a Pennsylvania gun owner from a family of hunters, was one of many consumers trying to buy a Bump Fire bump stock this week.
“I don’t even have the gun for it, but I want the stock just to have it down the line,” he said. “I just like the idea of them and want to see how it feels and if it’s worth it — for $100, it’s almost not a bad investment to buy it, try it out and sell it if I don’t like it.”