Via Guns.com: 

The Department of Justice unveiled a regulation late last week “effectively banning” bump stocks.

Attorney General Jeff Sessions said the new rule clarifies bump stocks fall under the definition of “machine gun” as it pertains to the National Firearms Act of 1934 and the Gun Control Act of 1968...

...“In this proposed rule, the department accordingly interprets the definition of ‘machine gun’ to clarify that all bump stock-type devices are ‘machine guns’ under the GCA and NFA because they convert a semiautomatic firearms into a firearm that shoots automatically more than one shot, without manual reloading, by a single function of the trigger,” the DOJ said Friday.

The official notice of proposed rulemaking will be published in the Federal Register, Sessions said.

In the meantime, gun rights groups have already promised a legal challenge.

Read more here.