The Sputter Gun was a modification of the British Sten Submachine Gun, designed to circumvent then-existing U.S. laws defining a machine gun. Lacking a trigger, the Sputter Gun was designed to, upon release of the bolt, fire multiple rounds until all ammunition was expended. The Sputter Gun was, however, reclassified as a machine gun by the ATF.
The 'shoots automatically' definition covers weapons that will function automatically. The 'readily restorable' definition defines weapons which previously could shoot automatically but will not in their present condition. The 'designed' definition includes weapons which have not previously functioned as machineguns but possess specific machinegun design features which facilitate automatic fire by simple alteration or elimination of existing component parts.