The US Department of Justice has formally submitted a regulation to ban "bump stocks", a modification to high-capacity rifles that lets them fire like an automatic weapon.
President Donald Trump signed a memorandum in February directing the department to make the regulatory change, which must now be approved by the Office of Management and Budget before it is published and subject to a commentary period.
The move does not require congressional approval, allowing the administration to sidestep what could have been insurmountable pressure from pro-gun groups such as the National Rifle Association that have worked to erode changes in firearm laws after mass shootings in Florida and Nevada.
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The NRA supported more regulations on bump stocks but has not endorsed Mr Trump's ban and said previously it was awaiting the publication of the regulation before rendering judgment.
The NRA could not immediately be reached for comment.
Shooter Stephen Paddock was in possession of a "bump stock" after a shooting rampage in Las Vegas in October 2017 that left 58 people dead and more than 850 injured.