I doubt that GOA/GOF will win (succeed in overturning the NFA) based on the idea that a tax cannot be levied on a right. One clearly has a right to own real property yet real property is taxed in almost all states. IMHO, they would stand a better chance with the argument that if X is legal under state law then one has a reasonable expectation that federal prosecution would be waived - as is the case for marijuana production, sales and possession in many states.
I've suggested this in the past:
Heller: the Second Amendment protects firearms in common use for lawful purposes.
Caetano: The Court has held that “the Second Amendment extends, prima facie, to all instruments that constitute bearable arms, even those that were not in existence at the time of the founding,” District of Columbia v. Heller, 554 U. S. 570, 582 (2008), and that this “ Second Amendment right is fully applicable to the States,” McDonald v. Chicago, 561 U. S. 742, 750 (2010)
As the foregoing makes clear, the pertinent Second Amendment inquiry is whether stun guns are commonly possessed by law-abiding citizens for lawful purposes today. The Supreme Judicial Court offered only a cursory discussion of that question, noting that the “ ‘number of Tasers and stun guns is dwarfed by the number of firearms.’ ” 470 Mass., at 781, 26 N. E. 3d, at 693. This observation may be true, but it is beside the point. Otherwise, a State would be free to ban all weapons except handguns, because “handguns are the most popular weapon chosen by Americans for self-defense in the home.” Heller, supra, at 629.
The more relevant statistic is that “[h]undreds of thousands of Tasers and stun guns have been sold to private citizens,” who it appears may lawfully possess them in 45 States. People v. Yanna, 297 Mich. App. 137, 144, 824 N. W. 2d 241, 245 (2012) (holding Michigan stun gun ban unconstitutional); see Volokh, Nonlethal Self-Defense, (Almost Entirely) Nonlethal Weapons, and the Rights To Keep and Bear Arms and Defend Life, 62 Stan. L. Rev. 199, 244 (2009) (citing stun gun bans in seven States); Wis. Stat. ?941.295 (Supp. 2015) (amended Wisconsin law permitting stun gun possession); see also Brief in Opposition 11 (acknowledging that “approximately 200,000 civilians owned stun guns” as of 2009). While less popular than handguns, stun guns are widely owned and accepted as a legitimate means of self-defense across the country. Massachusetts’ categorical ban of such weapons therefore violates the Second Amendment.
ATF records: silencers and SBRs possessed for lawful use exceed "hundreds of thousands".