Good luck trying to get all semi-automatics classified as NFA items.
Similarly with requiring an FFL for any transfer: you have to be in the "business" of selling firearms, which would mean it must be above the "hobby" level as the IRS classifies it.
The biggest direct threat is if they tried to pull an FFL holder's license if he didn't treat an "assault weapon" as an NFA item, but even that I think would be dispensed with in the courts fairly quickly.
I think the go after dealers and manufacturers with product liability law suits. Basically sue them out of existence. Keep what you got.