Let's analyse that for a second.
We'd have to start by ignoring the fact that they couldn't find a statutorily valid way to call these a machine gun for over a decade of trying. The statute's language simply can't define one as a machine gun.
So skipping that and just saying a bump stock IS a machine gun, then you would need to do an analysis on whether a deactivated bump stock was "readily convertible" into a bump stock. - Like any collapsible stock is if you hold the detente open, and mount the pistol grip to the stock.
This whole thing is untenable from the perspective of applying a law and then enforcing it. It only "works" on the basis of " well this is what our agency is going to do, and all the other agencies &/ interests with the ability to object have agreed not to call us on it."
NRA and Donny boy sold us out hard. GOA hasn't. Who knows what a judge will say, but if the judge actually applies legal analysis to it, I don't see how he can conclude that there is any authority to ban bump stocks which doesn't equally apply to all semi autos, and further, that the actual law does distinguish semi autos, so that can't be valid.