The Trump administration has banned the use of bump stocks, devices that let rifles fire like machine guns, after promising to do so earlier this year.
The final date to destroy or turn in the devices is 21 March, said White House Press Secretary Sarah Sanders.
The push to ban bump stocks followed the deadly mass shootings in Las Vegas in October 2017 and Parkland, Florida in February.
Pro-gun advocates have said they are prepared to fight the rule in court.
Acting Attorney General Matthew Whitaker signed the new regulation on Tuesday, and it is expected to be published in the Federal Register on Friday.
Bump stocks, or slide fire adapters, allow semi-automatic rifles to fire at a high rate, similar to a machine gun, but can be obtained without the extensive background checks required of purchasing automatic weapons.
Las Vegas gunman Stephen Paddock used a bump stock to fire rapidly into the crowd, killing 58 last year.
Following the Las Vegas shooting, lawmakers began discussing a ban on the devices.
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In February, 17 people were shot and killed at a high school in Parkland, Florida, reigniting the gun control debate, though bump stocks were not used in that attack.
Shortly after, President Donald Trump directed the Department of Justice to look into changing regulations so that bump stocks would be classified as machine guns, which are illegal to possess in most cases.
The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) had previously ruled that bump stocks did not qualify as machine guns and thus would not be regulated.
What do gun advocate groups say?
The Gun Owners of America lobby issued a statement on Tuesday saying they have prepared a lawsuit against the ATF and the justice department on behalf of the “half a million bump stock owners” forced to part with their “valuable property”.
“Agencies are not free to rewrite laws under the guise of ‘interpretation’ of a statute, especially where the law’s meaning is clear,” said Erich Pratt, executive director of Gun Owners of America.
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The National Rifle Association (NRA), which had called for a review of bump stock devices after the Las Vegas shooting, said they were “disappointed” with the new ban, according to the Associated Press.
An NRA spokeswoman said the government should offer amnesty to current owners.
The Trump administration is ready for any lawsuits, officials told reporters on a call earlier on Tuesday, according to US media.
What are bumpstocks?
Since 1986, it has been relatively difficult for civilians to buy new, fully automatic weapons, which reload automatically and fire continuously as long as the trigger is depressed.
It is also illegal to modify the internal components of semi-automatic rifles – which typically manage about 60 aimed shots per minute – to make them fully automatic.
Gun owners can instead legally buy accessories to increase the rate of fire, like the bump stock.
Bump stocks harness a rifle’s recoil, They replace the weapon’s stock, which is held against the shoulder, and allow the rest of the rifle to slide back and forward with every shot despite having no mechanical parts or springs.
The motion makes the trigger collide with, or bump, the shooter’s finger as long as they apply forward pressure with the non-shooting hand and rearward pressure with the shooting hand.
Administration officials said that the devices are not extremely common, but there are probably tens of thousands nationwide, US media reported.