WASHINGTON -- Democrats in Congress will try to outlaw devices that can turn semi-automatic weapons into a close approximation of machine guns, days after a Las vegas gunman used such "bump stocks" to make it easier to mow down concertgoers at an outdoor festival, killing 58 and injuring about 500.

Ohio U.S. Sen. Sherrod Brown, a Democrat, said he will back the bill, introduced Wednesday by Sen. Dianne Feinstein, a California Democrat. More than 20 other Democrats have signed on as well.

"I have always respected the rights of hunters, collectors, and other law-abiding gun owners, and no one intends to take away their guns," Brown said. "But when one man can slaughter more than 50 innocent people in a matter of minutes, it's clear something has to be done to protect Americans from gun violence."

The bill, while drawn more narrowly than past efforts to restrict assault weapons, faces uncertainty in both the Senate and House because of past opposition of gun control from majority Republicans. But a number of Republicans signaled Wednesday the Las Vegas tragedy could mark a turning point.

So could the narrow nature of the bill. It would not outlaw any weapons. But it would outlaw sale of devices that attach to the body of semi-automatic weapons and enable them to operate somewhat similarly to machine guns, or fully automatic weapons, which are illegal without a special license.

Semi-automatic weapons fire at a rate of 45 to 60 rounds per minute. A user, however, can buy an inexpensive bump stock and shoot at a rate of 400 to 800 rounds a minute.

"Automatic weapons have been illegal for more than 30 years, but there's a loophole in the law that can be exploited to allow killers to fire at rates of between 400 and 800 rounds-per-minute," Feinstein said. "The only reason to fire so many rounds so fast is to kill large numbers of people. No one should be able to easily and cheaply modify legal weapons into what are essentially machine guns."

Ohio Sen. Rob Portman, a Republican, has not decided on the bill, said his spokesman, Emily Benavides.

"The tragedy in Las Vegas was horrific," Benavides said. "We will do our due diligence on this legislation and review it."

The "bump stock" - a device legally sold in the US that can make a semi-automatic weapon mimic the firepower of fully automatic machinegun pic.twitter.com/PuXu8kpRIY — AFP news agency (@AFP) October 4, 2017

Gun control is always a tough sell in Congress because of Republicans who hold the Second Amendment sacrosanct. Every tragedy and attack, from the 2012 shooting of 20 elementary school children in Newtown, Connecticut, to the killing of 49 people at the Pulse nightclub in Orlando, Florida, in 2016, brings calls from Democrats to restrict access and sale of certain firearms. Republicans successfully push back, citing the potential for gun owners and sportsman to be denied their constitutional rights.

Portman, for example, opposed the bill after Newtown to restrict transfer of firearms at gun shows and through social media and elsewhere online, where buyers can avoid more scrutiny than the background checks conducted at brick-and-mortar gun stores. Portman said the bill could unfairly impinge on innocent transfers, such as giving a hunting rifle to a second cousin.

His view was not unique.

When Democrats in 2015 tried to ban people on a federal list of suspected terrorists from being able to buy firearms -- they were already barred from getting on a plane -- Republicans including Portman objected. They said the measure could deny Second Amendment rights to someone whose name was accidentally on such a list and posed no actual risk. Portman said later -- after the Pulse shootings -- that he could support such a bill as long as people who were on the list by mistake could remove themselves from it. Critics accused him of flip-flopping in an election year.

The Second Amendment says: "A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed."

Gun right activists and gun control advocates parse the language differently. The U.S. Supreme Court has said the Second Amendment does not bar some regulation. But debate rages over how far the regulation can go, and congressional Republicans, many with political backing from the NRA, are loath to push the question.

Feinstein's bill could provide a new test of political will -- and a test of what gun-control limits might finally be acceptable after the worst mass shooting in the nation's modern history. While some Republicans were reported as still opposed to restrictions, several Republican leaders said they were interested in at least learning more.

The subject is "certainly something that's got my attention and I think we ought to get to the bottom of it," Sen. John Cornyn of Texas, the second-ranking Senate Republican, told Politico.