WASHINGTON — The Biden administration said on Friday that it would crack down on the sale of firearm accessories used to convert short-barreled semiautomatic weapons into long rifles, part of a stepped-up effort to address mass shootings.

A final rule released by the Bureau of Alcohol, Tobacco, Firearms and Explosives will require about three million gun owners to apply for a permit within 120 days to use the accessories, known as stabilizing braces. After that, they will have to pay a $200 fee. New buyers of braces will have to pay the tax immediately.

“In the days of Al Capone, Congress said back then that short-barreled rifles and sawed-off shotguns should be subjected to greater legal requirements than most other guns,” the director of the A.T.F., Steven M. Dettelbach, said in announcing the rule alongside Attorney General Merrick B. Garland. “The reason for that is that short-barreled rifles have the greater capability of long guns, yet are easier to conceal, like a pistol.”

The rule comes on the heels of the government’s decision to strictly enforce another federal regulation, this one intended to limit the availability of homemade, untraceable firearms known as ghost guns.

Both measures, which rely on interpretations of existing federal law, are facing legal challenges from gun rights groups that claim they violate the constitutional right to bear arms.

Under federal law, possession of sawed-off shotguns and other short-barreled rifles — favored by criminals because they combine easy concealment with lethal firepower — is illegal. As its name indicates, a removable stabilizing brace allows assailants to leverage shorter-length weapons against their shoulder, providing a more stable platform for firing rapid bursts of deadly fire.

Mr. Garland said the rule was intended to alert firearm manufacturers, dealers and individual gun owners that they “cannot evade these important public safety protections simply by adding accessories to pistols that transform them into short-barreled rifles.”

The rule will become effective over the next week after it is published in the Federal Register. A preliminary version was submitted earlier this year and officials said they made one substantial change — eliminating a checklist to determine if the rule applied to a gun — after receiving more than 230,000 comments from citizens, many of them gun owners opposed to the regulation.

Officials with the National Rifle Association have criticized the new rule, saying it would adversely affect disabled veterans and other people who require braces to fire handguns.

In late December, the Biden administration closed a major loophole in a new federal rule, finalized over the summer, to regulate ghost gun components.

The A.T.F., under pressure from gun control activists, directed vendors who sell partly finished frames of Glock-style handguns — the pistol grip and firing mechanism — to treat them like fully completed firearms, which are subject to federal regulations.

The guidance could severely restrict the sale of unregulated and untraceable “80 percent” frames and receivers that have been linked to thousands of crimes, a top goal of the gun control movement. Such parts require only simple alterations to become operational.

This month, an appeals court in New Orleans suspended implementation of another rule, proposed during the Trump administration, that blocks the sale of another accessory, “bump stocks,” used by the man who killed 60 people in the mass shooting in 2017 in Las Vegas.