Suppose they're just removing the checkbox to apply to make an MG, thus making it impossible to attempt to pay the tax (as there would be no valid form to make MGs). What legal recourse would we have?
Essentially, as I understand it, you'd have to manufacture a machinegun, get caught, get prosecuted, and then you'd be able to raise as a defense during your trial the fact that they wouldn't let you pay the tax. And there IS some precident to the effect that you can't be convicted for not paying a tax if they won't let you pay it.
You'd have to do it in a state which didn't have a state law against manufacturing a machine gun, as otherwise you'd be prosecuted under state law, which the commerce clause ruling doesn't apply to, be rendered a felon, and couldn't own ANY guns.
So the real question is whether there are any states in the 9th circuit which don't have laws against manufacturing/posessing a machinegun. And you'd probably want it to be a state which didn't under state law require machineguns to be registered with the feds, because they
might be able to nail you on a state charge of non-registering even if you could prove the feds wouldn't let you register it, since you DID have the option of not manufacturing it in the first place.
My hunch is that they issued this ruling knowing in advance that there was no way for anyone to take advantage of it. It was just a way of tweaking the Supreme court's nose about the REAL implications of it's commerce clause reasoning in the Lopez decision.