Here in Massachusetts, it’s up to the jury to decide whether a particular weapon is or isn’t a machine gun. See Commonwealth v. Batholomew, 326 Mass. 218, 222 (1950).
Nevertheless, the legislature has expressly defined the term “machine gun.”
According to M.G.L. c. 140, Sec. 121, a machine gun is
a firearm, loaded or unloaded, which may automatically discharge more than 1 shot by a continuous activation of the trigger, whether originally manufactured as such or modified by automatic conversion, including through the use of an automatic part or any firearm, loaded or unloaded, which has been modified by automatic conversion to alter or increase its rate of fire to mimic automatic fire; provided, however, that ”machine gun” shall include a submachine gun.
A similar description is given in Model Jury Instruction 7.635
A machine gun is a weapon of any description, by whatever name known, from which a number of shots or bullets may be rapidly or automatically discharged by one continuous activation of the trigger, including a submachine gun or a weapon with a bump stock or trigger crank.
Unlawful possession of a machine gun is prohibited by M.G.L. c. 269, Sec. 10(c) which also bans illegal ownership of “a sawed-off shotgun, automatic part, bump stock, rapid-fire trigger activator or trigger modifier.”
Violations are M.G.L. c. 269, Sec. 10(c) carry a mandatory minimum sentence of 18 months in prison.