Except the Miller case was “legal” because sawed-off shotgun was deemed not military useful, which allowed the tax to stay in place. While the court ignored the military useful automatic weapon tax and SBR tax.
Funny that.
And a direct challenge to Miller is something that should be done. How that case was handled is utterly indefensible under modern jurisprudence. One side not being represented by a lawyer and explicitly saying that they trust the government to present both sides makes it a farcical case.
I would say challenge the NFA with regards to suppressors but that runs into the thorny issue of whether or not they count as "arms". If they aren't then the 2nd Amendment doesn't cover them and interstate commerce effectively justifies their treatment (regardless of how stupid that treatment is).
The SBR restrictions are a direct challenge to Miller, which gives justification for SCOTUS to take the case, are squarely covered under the 2nd, and are effectively impossible to justify given pragmatic realities.
It's also, politically speaking, a lot easier for the court to do than anything touching on machine guns would be.
Honestly, SCOTUS is really unlikely to take a direct challenge to the machine gun registry being closed at the moment. They will want a more solid body of cases to help justify it, for more time to pass, and probably for a circuit split on the issue.
If you go after SBR's then the existence of Miller argues strongly in favor of SCOTUS taking the case and you are likely to get an opinion that the government can't restrict entire categories of firearms in what is essentially an arbitrary and capricious manner. Get that opinion and it gets a lot easier to rule that the closed registry is just such an arbitrary and capricious restriction.