I'm guessing if they were to attempt to regulate bump fire devices it wouldn't be by literally redefining the machine gun.
Also, specifically banning or regulating bump "stocks" is too narrow to be effective. It wouldn't cover rubber bands or trigger cranks or extra springy butt pads or hamster wheels.
They would probably fall back on the type of weasel wording previously used like "parts when placed in close proximity that serve no useful purpose other than to" or "parts designed or redesigned or intended to" ... "increase rate of fire yada yada, blah blah", maybe (begrudgingly) something about "mechanical advantage".
Something like this might be broad enough to hang someone by the nuts for parts either external or internal that facilitate the emptying of a magazine quickly.
The problem is it gets sticky when attempting to define what is a regulatable ( <- real word? ) increase in rate of fire because of this guy ... 480 rounds per minute with an unmodified wheelgun and no external parts.
How fast is too fast to pull a trigger? A device that provides a mechanical advantage to increase rate of fire and yet does not out shoot a guy squeezing the bang switch with his booger hook would be an embarrassment in logical aptitude, as has been demonstrated in the past, not quite beyond the ability of the of the BATFE.
The godfather of speed on how to shoot fast.https://www.youtube.com/watch?v=WyIq9FdTgwM