"On December 18, 2018, Acting Attorney General Matthew Whitaker announced that the Department of Justice has amended the regulations of the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), clarifying that bump stocks fall within the definition of “machinegun” under federal law, as such devices allow a shooter of a semiautomatic firearm to initiate a continuous firing cycle with a single pull of the trigger."
Understand that this is an administrative ruling on a specific item and they have declared that it now falls under the definition of machine gun. This is not a change to the definition of what a machine gun is, in Federal law.
While it is more than a bit of a stretch, it is arguably allowable if one accepts the "initiates a continuous firing cycle" (where in reality the trigger is pulled over and over) as being covered under the law's definition of firing more than a single round with a single pull of the trigger.. Personally, I think it's a BS ruling, and is outside the definition in law, but until/unless someone with deep enough pockets and a stake in the matter takes it through the court process (years worth) the BS will stand until a court rules otherwise.
This is a far, far cry from claiming that SEMI AUTOS are, or should be machineguns. I do not believe there is any way the ATF can amend or change any of its rulings to cover that. Doing so simply grossly exceeds their lawful authority.