In the immediate aftermath of the deadliest mass shooting in modern United States history, there was a fevered pitch to ban bump stocks, the device that allowed the shooter's semiautomatic rifles to mimic the rapid fire of machine guns.
With that push stalled at the federal level, a handful of states and some cities are moving ahead with bans of their own.
Massachusetts and New Jersey – two states led by Republican governors – as well as the cities of Denver and Columbia, S.C., have enacted laws prohibiting the sale and possession of the devices, which were attached to a half-dozen of the long guns found in the hotel room of the Las Vegas shooter who in October killed 58 people and injured hundreds more attending a nearby outdoor concert.
A little more than a dozen other states are also considering bans on bump stocks.
Gun-control advocates say the push fits a pattern in gun politics: inaction in Washington that forces states to take the lead. Gun-rights advocates call it a knee-jerk reaction that will do little to stop bad guys from killing, and vow a legal challenge.
For Zach Elmore, the issue is deeply personal. His sister among those wounded in the Las Vegas attack. He finds hope in the statewide and local efforts to ban bump stocks.
"Hopefully it's the start of a big movement versus just a flash in the pan," Mr. Elmore said. "Obviously you cannot legislate [against] evil, but you can legislate the things with which bad people will use to perpetrate evil."
The devices were originally intended to help people with disabilities and were little known and little sold until the Las Vegas shooting. They fit over the stock and pistol grip of a semi-automatic rifle and allow the weapon to fire rapidly, some 400 to 800 rounds per minute, mimicking a fully automatic firearm.
The federal Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) reviewed the devices and approved them in 2010, ruling they did not amount to machine guns that are regulated under the National Firearms Act that dates to the 1930s.
Richard Vasquez was assistant chief of the ATF's technology bureau and led the review of bump stocks. He stands by the agency's 2010 ruling, which relied on a key difference between semi-automatic and fully automatic firearms: While semi-automatic firearms require a separate and independent pull of the trigger to fire a bullet, fully automatic firearms can fire multiple rounds with a single trigger pull. ATF determined bump stocks didn't convert a semi-automatic firearm into one that is fully auto.
"It's a proper determination. Everybody wants to jump the gun and say that ATF made a mistake," said Mr. Vasquez, who left the agency in 2014. "ATF didn't make a mistake."
The federal agency is reviewing its ruling, something Vasquez and others caution is a dangerous move without Congress first changing the law to specifically make such devices illegal.
"If ATF is allowed to write a regulation to change the definition of this device, instead of a law, it's going to give all government agencies authority to change their regulations, which could affect us in a wide variety of ways," Vasquez warned.
Legislation in Congress has remained in limbo despite early signs from a bipartisan mix of lawmakers and advocates who voiced alarm that such a device was deemed legal and on the market. Even the National Rifle Association sounded open to great regulation of bump stocks.
Joyce Malcolm, a professor at George Mason University's Antonin Scalia School of Law, said the bans likely would withstand a legal challenge, but she wonders about more practical matters: How might they be enforced?
"I don't see a real constitutional issue. I just wonder about actually getting these devices out of circulation for people who already have them," she said.
It's not known how many of the devices are in circulation. At the time of the Las Vegas shooting, gun dealers said they rarely had customers wanting to buy one – but then couldn't keep up with the demand in the days and months afterward. The leading manufacturer of bump stocks, Texas-based Slide Fire Solutions, briefly stopped selling them, posting a notice on its website that it had suspended sales to keep up with demand. It has since resumed sales.
Massachusetts, which has some of the nation's strictest gun laws, enacted its ban a month after the Las Vegas shooting, pushed through a Democratic-controlled legislature and signed into law by a Republican lieutenant governor. New Jersey followed suit last month with a measure signed into law by outgoing Republican Gov. Chris Christie. About a dozen other states are considering similar bills.
"This is a very familiar story. The Congress cowers in the face of the NRA, and the states act," said John Feinblatt, president of Everytown for Gun Safety, which advocates for tougher gun laws.
Connecticut, home to some of the world's most legendary gun makers, is among the other states considering bans.
Rep. William Tong, a Democrat and House chairman of the Connecticut legislature's Judiciary Committee, called it a commonsense measure that puts public safety first.
"The federal government has demonstrated that, given the composition of the Congress right now, it's not possible to pass commonsense gun laws," he said. "Connecticut can't wait."
Gun-rights advocates say the efforts undermine the Second Amendment and will do little to stop criminals.
"Anti-gun Democrats love to ban things. That is their answer," said Erich Pratt, executive director of Gun Owners of America, who added bump stocks have rarely, if ever, been used in a crime before the Las Vegas shooting. "This is their chance to say, 'Hey, we're doing something. Look at us. We're doing something.' It's not going to do anything."
This story was reported by The Associated Press.