WASHINGTON (Corrects fifth paragraph of this Oct. 1 story to say not all machine guns are illegal in the United States.)

WASHINGTON (Reuters) - U.S. President Donald Trump said on Monday his administration is just a few weeks away from finalizing a regulation that would ban so-called bump stocks, devices that allow semi-automatic weapons to fire like machine guns.

"We're knocking out bump stocks," Trump said at a White House news conference. "We're in the final two or three weeks, and I'll be able to write out bump stocks."

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A year ago in Las Vegas, gunman Stephen Paddock used bump stocks on 12 of his weapons in a mass shooting that killed 58 people and wounded hundreds.

Authorities said his ability to fire hundreds of rounds per minute over the course of 10 minutes from his perch in a 32nd-floor hotel suite was a major factor in the high casualty count.

While machine guns manufactured after May 1986 are outlawed in the United States, bump stocks are not.

U.S. Attorney General Jeff Sessions said in March the Justice Department was proposing a rule that would effectively ban the devices. In February, Trump had signed a memorandum directing the department to make the regulatory change.

The change required a public comment period before taking effect.

"We are now at the final stages of the procedure," Trump said.

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(Reporting by Steve Holland; Writing by Tim Ahmann; Editing by Frances Kerry)