Donald Trump ordered US attorney general Jeff Sessions to “propose regulations to ban all devices to turn legal weapons into machine guns” today. He was referring to so-called bump stock devices which can modify a semi-automatic rifle to fire as many as 800 bullets per minute.

Trump’s statement comes amid rising calls for gun control in the US, after 17 students were killed last week in Parkland, Florida by a former classmate armed with a semi-automatic weapon. But the change Trump is proposing wouldn’t have had any impact on the Florida shooting, which didn’t involve a bump stock.

More broadly, the Department of Justice can’t just ban such devices, its own officials have said in recent months.

Trump laid the responsibility today directly on Sessions during a press conference in the White House. “I expect that these critical regulations will be finalized, Jeff, very soon,” Trump said, speaking to the attorney general:

The Department of Justice’s Bureau of Alcohol, Tobacco, and Firearms (ATF) has been reviewing whether adding a bumpstock makes a gun a “machine gun” since last December, after a gunman armed with multiple weapons outfitted with bumpstocks killed 58 people and injured over 800 in Las Vegas. Fully automatic weapons, or machine guns, are illegal in the US, but semiautomatic weapons like the AR-15 used in the Florida shooting are not.

“We will go through the regulatory process that is required by law and we will be attentive to input from the public,” Sessions said at the time. Of the over 36,000 public comments the ATF got on the proposal, 85% were against reclassifying bumpstock-modified weapons as “machine guns,” The Trace found.

Since then, DOJ and ATF officials have said publicly and privately that banning bump stocks could not be achieved unilaterally. They argue that Congress needs to pass legislation in order to reclassify bumpstocks in the US, the New York Times reported (paywall). The ATF “could not find a way to classify it as a machine gun,” in 2010, a former official told The Trace.

After a mass shooting in Las Vegas last year, a bill to ban bumpstocks was re-introduced in October. However, this stalled after speaker of the House Paul Ryan said the ATF, not Congress, should be responsible for the issue. It’s unclear whether Trump’s support for the idea would be enough to get legislation passed in Congress this time around.

Meanwhile, Florida state legislators today voted down a bill that would ban assault rifles, as students from the Parkland high school looked on.