Sputter Gun

The Sputter Gun was a US modification of the British Sten Submachine Gun, designed to circumvent then-existing U.S. laws defining a machine gun. The Sputter Gun, lacking a trigger, was designed to fire multiple rounds upon release of the bolt, until all ammunition was expended. The Sputter Gun was, however, reclassified as a machine gun by the ATF.

Background
In 1985, the ATF became aware that William M. York, doing business as York Arms Co., was selling a modified version of a Mk.II Sten that was capable of fully automatic fire. York advertised the gun as a firearm for those "who want the fun and excitement of owning and firing a fully automatic firearm without the government tax and red tape." The ATF instructed York to recall firearms already sold and issue refunds. This was unnecessary, however, as none had been sold.

Reclassification
The ATF originally defined a machine gun as any firearm capable of firing multiple rounds by a single trigger action. This was changed, in light of the Sputter Gun, to read:

"That 'shoots automatically' definition covers firearms that will function automatically. The 'readily restorable' definition defines firearms which previously could shoot automatically but will not in their present condition. The 'designed' definition includes firearms 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."