The Trump administration is banning bump stocks, the firearm attachment that allows a semi-automatic weapon to shoot almost as fast as a machine gun.
The devices, also known as slide fires, came under intense scrutiny after they were used by the gunman who opened fire on a country music concert in Las Vegas last year, killing 58 people.
The massacre touched off a public outcry, including from some lawmakers, for the accessories to be banned.
Under a new federal rule announced Tuesday by the Justice Department, bump stocks will be redefined as "machine guns" and therefore outlawed under existing law.
The new regulations, which were signed by acting Attorney General Matthew Whitaker, will take effect 90 days after being published in the Federal Register. A Justice Department official said that would likely happen Friday.