The U.S. Justice Department says it has started the process to ban bump stock devices, under federal firearms regulations.
On Friday, U.S. Attorney General Jeff Sessions said bump stocks fall within the legal definition of "machine guns".
The devices allow semi-automatic guns to fire like automatic weapons, and were used in last year's Las Vegas massacre.
For months, President Trump has urged the Justice Department to pursue the bump stock ban.
He tweeted on Friday that "We will BAN all devices that turn legal weapons, into illegal machine guns."