Right on with the semi-auto implications. The "rule" relies on the the definition of "machinegun" which is mechanical in nature (cyclical rate of fire), not shortening the reset, not shortening the length of pill, not…anything other than one pull of the trigger causes the weapon to fire until the trigger is released. The bump stock doesn't increase the cyclical rate of fire. The shooter is still "pulling" the trigger each time. From a purely legal perspective, the rule shouldn't stand. No deference should be given by the courts. If Congress doesn't like it, Congress can change it. It's beyond the purview of the DOJ or any agency. That being said, a lawsuit has already been filed.
https://www.firearmspolicy.org/lawsuit-challenging-trump-bump-stock-ban