It's been a few years since I read through some of the relevant opinions, but IIRC, the deprivation of property rights has to be complete and also permanent before it qualifies. Since the value of the plastic butt stocks in question has become zero, the company may have a case...
I think bump stocks are stupid, but I don’t think they should be illegal any more than full auto. I wish full auto were like semiautomatic for purchase, but that’s another battle. At least you can get a full auto if you want to pay the tax.
Given the sequence of events, I think the bump stock company should be compensated for govt taking. It should come from ATF budget.