Here's an interesting read: the supreme court dismissed an indictment of a unregistered machine gun prosecution because the same base defense for the machine gun case can be applied to an active case involving the legality of obamacare. The tldr is a 2012 case re-legitimized the the power of the NFA because it is a tax, yet the government won't collect the tax to register a post 86 machine gun. The ACA also doesn't collect tax revenue anymore, and is being challenged on its legality on the same argument. Not sure what to make of this.

DOJ Dismisses Indictment of Machine Gun Prosecution to Preserve Its Obamacare Arguments
Obamacare’s individual mandate can no longer be sustained as a tax because Congress zeroed out any revenue, which benefits the target of an unrelated unconstitutional use of the taxing power.
www.cato.org