Stephen Paddock, the gunman who killed 58 people and wounded hundreds of others in Las Vegas in October had at least 12 rifles fitted with bump stocks, authorities have said.

Trump signed a memorandum recommending that Attorney General Jeff Sessions propose regulations that would declare that bump stocks are illegal because they effectively turn legal semi-automatic weapons into outlawed machine guns.

President Donald Trump announced Tuesday that he has recommended that "bump stocks" — devices that let semi-automatic weapons fire hundreds of rounds per minute — be banned.

Trump's announcement came six days after a gunman killed 17 people, 14 of them students, at a high school in Parkland, Florida, while armed with an AR-15 assault rifle.

"We cannot merely take actions that make us feel like we are making a difference. We must actually make a difference," Trump said at a White House event honoring first responders.

"After the deadly shooting in Las Vegas, I directed [Sessions] to clarify whether certain bump stock devices like the one used in Las Vegas are illegal under current law," Trump said.

"That process began in December, and just a few moments ago I signed a memorandum directing the attorney general to propose regulations to ban all devices that turn legal weapons into machine guns," Trump said.

"I expect that these critical regulations will be finalized, Jeff, very soon."

The proposed regulators would first have to be published in the Federal Register and be subject to public comment before they could be adopted.

Semi-automatic weapons require a shooter to pull the trigger each time to fire a single round.

But when those weapons are outfitted with a bump stock, the gun's recoil energy is used to "bump" the trigger into the shooter's finger, making it fire much faster.

That makes the weapon more akin to machine guns, which are largely banned in the United States.

Read Trump's memo: