Has NFA ever been tested in court? I don’t think it has.
Yes, it has.
It was tested in 1939 in the infamous Miller case, a total set-up, one of the worst cases in Supreme Court history.
The decision upheld the individual right of the Second Amendment, but was very muddy. Read about it here.
It was tested in the 1939 “US v Miller” case. Here is an extensive write-up on the case: http://rkba.org/research/miller/miller.html
This was a very screwed up case (and, I think, purposely so). “Screwed up” as in designed to favor the government position. First, Jack Miller was caught with a saw-off shotgun after the NFA had passed, and it had no tax stamp associated with it, so he was charged with violating the NFA. So he challenged, and won in the District Court (with the opinion by a fairly dependable liberal judge - which is very curious until you realize that the case would be appealed directly to the USSC). Then Miller was killed...and his lawyer didn’t represent his case in the USSC. Then the USSC said that “it was not within judicial notice...” that a sawed-off shotgun had any utility for militia purposes...when sawed-off shotguns were plentifully used during WW1, only 21 years earlier, by the US Army and Marines (they were known as “trench brooms”). Given that this type of gun was not of use in militia service, it was ruled to not be protected by the 2nd Amendment. Also, the argument upon which the government relied that I mentioned above, regarding taxation, and not outlawing of, full autos, sawed off shotties and other weapons, has been invalid since Title 18, Section 922 (o) was signed into law.
Given the above, the NFA is ripe for review before the USSC, and should be tossed out as violative of the Second Amendment to the U.S. Constitution.