The Trump administration announced this weekend that it will be banning bump stocks, a modification that increases a semiautomatic rifle's rate of fire.

"President Trump is absolutely committed to ensuring the safety and security of every American and he has directed us to propose a regulation addressing bump stocks," said Attorney General Jeff Sessions in a press release announcing the move. The Justice Department, he continued, had submitted draft rules declaring "that the National Firearms and Gun Control Act defines 'machinegun' to include bump stock type devices."

Given that federal law prohibits private individuals from possessing machine guns that were acquired after 1986, this would effectively ban anyone from owning the more recently invented bump stocks.

Bump stocks have been in the sights of lawmakers of both parties since a deadly shooting in Las Vegas last October, when 58 people were killed and hundreds more wounded at a country music concert. Twelve of the shooter's rifles had been modified with bump stocks.

Within days of that shooting, Sen. Diane Feinstein (D-Calif.) promised legislation banning the device. Reps. Carlos Curbelo (R-Fla.) and Seth Moulton (D-Mass.) introduced a bump stock ban a week later. Other Republican lawmakers, from House Speaker Paul Ryan (R-Wis.) to Sen. Rob Portman (R-Ohio) said they would be OK with some sort of ban. The NRA likewise endorsed "additional regulations" on the product.

States too have targeted bump stocks in in recent months, with Washington, New Jersey, Massachusetts, and Florida passing bans. Similar laws are working their way through the legislatures in Maryland and Illinois.

Following the Parkland shooting, Donald Trump also endorsed a federal ban, going as far as to personally direct Sessions in a press conference to implement new regulations as soon as possible.

One problem: Adding bump stocks to the definition of machine guns in existing federal law is legally dubious, as Reason's Jacob Sullum has noted.

According to statute, a machine gun is "any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger." But bump stocks fire just one shot per trigger function. Unlike a standard rifle stock, which is fixed to the barrel, a bump stock lets the barrel slide back and forth along the stock when firing, continuously "bumping" against the shooters trigger finger, increasing the rate of fire but still requiring one trigger pull per shot.

For this reason, the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) has declared multiple times that the feds tight restrictions on machine guns do not cover bump stocks.

Feinstein is also skeptical of the executive branch's power to ban bump stocks unilaterally. In a February statement, she said bluntly that the ATF "currently lacks authority under the law to ban bump stocks."

"If ATF tries to ban these devices after admitting repeatedly that it lacks the authority to do so, that process could be tied up in court for years, and that would mean bump stocks would continue to be sold." And indeed, Gun Owners of America has suggested it might pursue legal action against an executive bump stock ban.

The draft rules announced by Sessions will be reviewed by the Office of Budget and Management before being made public.