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.
Such devices enable a rifle to shoot hundreds of rounds a minute.
Speaking at the White House, Mr Trump said he had directed the justice department to propose a law to make the accessories illegal.
The gun control debate took on a new urgency after 17 people were killed at a school in Florida last week.
Students and parents affected by the massacre at Marjory Stoneman Douglas High School are planning a demonstration in the state capital of Tallahassee on Wednesday.
Some of them arrived on Tuesday in time to see the state legislature reject a proposed ban on assault weapons and large capacity magazines. However it will consider a package of more limited gun restrictions.
'We're not gonna back down'
By Paul Blake, BBC News, South Florida
In all around 100 students, spread across three buses, made the journey of more than seven hours to the state capital.
For many of these students, it is their first time to the capitol building in Tallahassee - much less to lobby their lawmakers over a national issue.
There is an excitement among the students, who sense that they may be on the cusp of starting a national movement.
"We're travelling to our state capital to make sure that none of these people that we grew up with - that we've known our whole lives - die in vain," Julia Salomone, 18, said confidently.
So how do the students expect the politicians to receive them? "Honestly, I feel like they're gonna be overwhelmed by us because they're going to see in our faces our determination, our commitment to this, because this is all we're fighting for right now," 16-year-old Rain Valladares said. "They're gonna look us in the face and see that we're not gonna back down."
What did Trump say?
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".
"The key in all of these efforts, as I said in my remarks the day after the shooting, is that we cannot merely take actions that make us feel like we are making a difference, we must actually make a difference," he said.
"We must move past clichés and tired debates, and focus on evidence-based solutions and security measures that actually work and that make it easier for men and women of law enforcement to protect our children and protect our safety."
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 Stephen Paddock, a 64-year-old gambler, who rained bullets on the crowd at an outdoor country music festival on the Las Vegas Strip in October.
More than 500 people were also injured in that attack, considered America's worst ever mass-shooting by a lone gunman.
Audio analysis found Paddock had been able to fire 90 bullets within 10 seconds from his room in the Mandalay Bay Resort and Casino.
Didn't Congress plan to ban the devices?
Both Democrats and Republicans agreed in the nationwide wave of horror following the Las Vegas attack that the sale of bump stocks should be outlawed.
However, a bill introduced to ban bump stocks, trigger cranks and other devices that can speed up a semi-automatic rifle's rate of fire has since stalled.
In December, the Bureau of Alcohol, Tobacco, Firearms and Explosives began considering a regulation for bump stocks, opening a process that drew more than 35,000 public comments.
Proposals to ban bump stocks have been put forward with mixed results at state level, including in South Carolina, Illinois, Washington and Colorado.
Previous efforts to introduce gun control measures in the wake of mass shootings in recent years have gone nowhere in the US Congress.
Might Trump consider any other gun control?
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 the Florida shooting.
"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. The age limit in most US states for buying an AR-15 is 18.
Over the weekend, Mr Trump said he was supportive of a bipartisan bill that seeks to improve the checks in place before someone can buy a gun.
Whether we are Republican or Democrat, we must now focus on strengthening Background Checks!— Donald J. Trump (@realDonaldTrump) February 21, 2018
That legislation intends to patch holes in the FBI's background check System, which processed more than 25 million gun ownership applications last year.
Failures in that database were exposed by the Florida shooting, allegedly committed by a 19-year-old gunman with a history of mental health issues. Bump stocks were not used in that attack.
Will this time be different?
Analysis by Anthony Zurcher, BBC News, Washington
Whether the bureaucratic review process just initiated by the president will amount to a comprehensive ban on bump stocks depends very much on how the yet-to-be-written regulation is crafted.
Still, it is 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 National Rifle Association, however, is not 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 Florida. If the past is any guide, attention will fade and the drive to enact new legislation will slow. Will this time be different?