Bump stocks were first invented under Bush II (the Akins Accelerator) and were briefly legal then. The ATF quickly realized exactly what it was that a junior examiner had approved and banned it as (surprise) a machine gun.
It was only under Obama that that ban was reversed by someone calling it ‘an assistive device for the physically disabled.’ It was the Obama BATF that was too stupid to realize what it was.
I can bump fire all day without a bump stock. It is not a machine gun because the trigger is pulled for each shot. And it has been used by returning vets with hand prosthesis in semi auto fashion