“The National Firearms Act, 26 U.S.C. 5845(b) defines “machine gun” to include any combination of
parts designed and intended for use in converting a weapon to shoot automatically more than one shot,
without manual reloading, by a single function of the trigger.”
Note the words “single function of the trigger”.
The bump stock facilitates rapid multiple funtions of the trigger so by the NFA definition does not make a firearm into a machine gun.