There is no loophole. ATF definition of a MG is per the NFA of '34. Ie., the federal government owns the legal definition.

The Akins Accelerator was ruled a MG b/c it incorporated a spring. The spring was determined to be the "automatic action" feature (Perlmutter letter). The Slide-Fire stock and Hell-Fire trigger devices do not incorporate a spring, so are not MGs per ATF. This information, and the ATF letters, are searchable.

You can bump fire about any semiauto by matching the elastic characteristics of the trigger mechanism to........a shoestring.....a rubber band......or some practiced hand position. ATF are well aware of this fact. Type "ATF shoestring letter" into your search engine if you think I'm kidding. Certain devices make the process simpler to execute in quasi-seamless fashion.