2. United States v. Miller, Haynes v. United States, United States v. Freed, and District of Columbia v. Heller were at least four times the court had an opportunity to address the NFA and failed to find it unconstitutional. There may be others, but I'm not a lawyer and my Googlefoo attention span is short. Not surprising, as the legislature back then knew they couldn't outright ban machine guns, silencers, short barreled shotguns/rifles, or destructive devices, the NFA was crafted as a tax scheme not a ban. An interesting attack on the NFA might be equating it to a poll tax, but I don't know how far that would take you.
3. Unless and until the Supreme Court grants cert on a challenge to an 'assault weapon' or 'high capacity' ban law, they are tacitly giving their approval, or at least are not outraged enough over the practice to grant cert.
I think I should have been more specific—the Supreme Court has only addressed the constitutionality of the NFA under the Second Amendment once, in Miller.
There was no Second Amendment challenge in Haynes or Freed. In Haynes, the Supreme Court held only that a registration requirement then in place in the NFA violated the 5th Amendment prohibition against self-incrimination. In Freed, the Supreme Court held that a subsequent change to the NFA cured the 5th Amendment violation found in Haynes. The Second Amendment was not mentioned in either case.
Heller did not involve the NFA at all. It was only mentioned in Heller’s discussion of the holding in Miller, where the Heller court narrowly read Miller to hold “only that the Second Amendment does not protect those weapons not typically possessed by law-abiding citizens for lawful purposes, such as short-barreled shotguns.”
The Supreme Court did not have an opportunity to address the NFA’s constitutionality under the Second Amendment in Haynes, Freed, or Heller because no Second Amendment challenge to the NFA was asserted in any of those cases. The Supreme Court, like any court, can only address issues that are placed before it by the litigants, other than issues of its own jurisdiction.