US President Donald Trump has signed an order to ban bump-stock devices, which were used by a gunman who killed 58 Las Vegas concert-goers last year.
Speaking at the White House, Mr Trump said he had directed the Department of Justice to propose a law to make the accessories illegal.
“We have to do more to protect our children,” he said, adding that he would discuss school safety this week.
Last week a 19-year-old killed 17 people at his former Florida school.
At an event on Tuesday recognising the bravery of law enforcement, Mr Trump said he had directed Attorney General Jeff Sessions to finalise new guidelines to declare bump stocks illegal “very soon”.
What are bump stocks?
The accessories can make semi-automatic rifles fire as rapidly as machine guns.
They can be bought for as little as $100 (£70) without the need for a criminal background check.
The device was used by a 64-year-old gambler who rained bullets on a crowd at an outdoor country music festival on the Las Vegas Strip in October last year.
More than 500 people were injured in that attack, considered America’s worst ever mass-shooting by a lone gunman.
Audio analysis found the attacker, Stephen Paddock, was able to fire 90 bullets within 10 seconds from his room in the Mandalay Bay Resort and Casino.
Experts say Paddock, who killed himself after the attack, would not have been able to harm so many people without the use of bump stocks.
Didn’t Congress plan to ban them?
Some lawmakers, including the most powerful congressional Republican, House of Representatives Speaker Paul Ryan, said they had never heard of the device until the Las Vegas massacre.
But both Democrats and Republicans agreed in the national wave of horror following the attack that the sale of the accessories should be outlawed.
Last November, Democratic Senator Dianne Feinstein introduced a bill to ban bump stocks, trigger cranks and other devices that can speed up a semi-automatic rifle’s rate of fire.
But that legislation has since stalled.
The National Rifle Association and several Republicans said it was up to the Bureau of Alcohol, Tobacco, Firearms and Explosives to regulate the accessories.
Just a month after the Las Vegas massacre, Congress seemed to slide into the inertia that has bogged down previous gun control efforts.
Proposals to ban bump stocks have been put forward with mixed results at state level, including in South Carolina, Illinois, Washington and Colorado.
Might Trump consider any other gun control plan?
On Tuesday, the White House signalled it was open to an age limit for people buying AR-15-type assault rifles, like the one used in a deadly school shooting last week in Parkland, Florida.
“I think that’s certainly something that’s on the table for us to discuss and that we expect to come up over the next couple of weeks,” said White House spokeswoman Sarah Sanders, when asked about a possible age requirement.
Over the weekend, Mr Trump said he is supportive of a bipartisan bill that seeks to improve the checks in place before someone can buy a gun.
That legislation intends to patch holes in the FBI’s National Instant Criminal Background Check System, which processed more than 25 million gun ownership applications last year.
Failures in that database were exposed by a shooting against a Texas church last year, and again with last week’s Florida high school shooting, allegedly by a teenage gunman with a history of mental health and behavioural issues.
Will this time be different?
Analysis by Anthony Zurcher, BBC News, Washington
A presidential memo directing a subordinate to complete a bureaucratic review process and promulgate a new regulation doesn’t sound like much, but in this case Donald Trump has given the gears of government a bit of a kick toward banning bump stocks.
Or, at the very least, banning “devices that turn legal weapons into machine guns” as he put it.
Whether that will amount to a comprehensive ban on bump stocks depends very much on how the yet-to-be-written regulation is crafted.
Still, it’s the closest the Trump administration has come to new firearm regulation after multiple mass shootings – and, perhaps, a reflection of the new pressure the president is feeling on the gun issue.
Congress, of course, could have stepped in at any point in the five months since the Las Vegas massacre and banned bump stocks without this contorted bureaucratic process. The NRA, however, isn’t keen on seeing a bipartisan coalition forming to enact any kind of new federal gun-control laws.
The question now is whether Mr Trump’s move relieves the pressure to “do something” after Parkland. If the past is any guide, attention will fade and the drive to enact new legislation will slow. Will this time be different?