Makes you wonder why Haynes and Freed didn’t attack the NFA on Second Amendment grounds.
I don’t think there was really much of a Second Amendment movement back then. Haynes and Freed came in 1968 and 1971, really at the very beginning of the gun control era, before gun owners really started pushing back in the late 70s and 80s. Also, Haynes and Freed were criminal defendants represented by court appointed counsel, not activist plaintiffs represented by activist lawyers like the plaintiffs in Heller and McDonald.
Also, Haynes was arrested with a short-barreled shotgun, and the regulation of short-barreled shotguns is exactly what was upheld in Miller. Freed was arrested with hand grenades—few even today argue that regulation of those offends the Second Amendment. Even if they had been motivated to raise Second Amendment challenges, that Supreme Court would have been unlikely to grant cert on those issues, and thank God for that. Could you imagine what the Second Amendment jurisprudence would look like if it was decided by the Warren Court under those facts?
Even the Second Amendment challenge in Miller was half-hearted at best. The Miller defendants’ lawyer did not even file a brief or attend oral argument because his clients weren’t paying him. That case was decided solely on the government’s brief, which helps explain its absurd reasoning.
The case would never even have made it to the Supreme Court today. It was there under a New-Deal-era law allowing the government to file a direct appeal to the Supreme Court if a district court held a law unconstitutional (those appeals have since been narrowed just to redistricting cases). The Supreme Court probably never would have granted cert or, had it done so, probably would have dismissed cert as improvidently granted under these circumstances.
There was not a lot of Second Amendment scholarship or activism until fairly recently, for the same reason there is not a lot of Third Amendment scholarship or activism today. It wasn’t until fairly recently that the government started violating it. I think the NFA contains multiple violations of the Second Amendment, but pre-1968, those violations were relatively mild and not enough to motivate a great deal of resistance.