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:
THE WHITE HOUSE
Office of the Press Secretary
FOR IMMEDIATE RELEASE
February 20, 2018
February 20, 2018
MEMORANDUM FOR THE ATTORNEY GENERAL
SUBJECT: Application of the Definition of Machinegun to "Bump Fire" Stocks and Other Similar Devices
After the deadly mass murder in Las Vegas, Nevada, on October 1, 2017, I asked my Administration to fully review how the Bureau of Alcohol, Tobacco, Firearms and Explosives regulates bump fire stocks and similar devices.
Although the Obama Administration repeatedly concluded that particular bump stock type devices were lawful to purchase and possess, I sought further clarification of the law restricting fully automatic machineguns.
Accordingly, following established legal protocols, the Department of Justice started the process of promulgating a Federal regulation interpreting the definition of "machinegun" under Federal law to clarify whether certain bump stock type devices should be illegal. The Advanced Notice of Proposed Rulemaking was published in the "Federal Register" on December 26, 2017. Public comment concluded on January 25, 2018, with the Department of Justice receiving over 100,000 comments.
Today, I am directing the Department of Justice to dedicate all available resources to complete the review of the comments received, and, as expeditiously as possible, to propose for notice and comment a rule banning all devices that turn legal weapons into machineguns.
Although I desire swift and decisive action, I remain committed to the rule of law and to the procedures the law prescribes. Doing this the right way will ensure that the resulting regulation is workable and effective and leaves no loopholes for criminals to exploit. I would ask that you keep me regularly apprised of your progress.
You are authorized and directed to publish this memorandum in the "Federal Register".
DONALD J. TRUMP