You are trying to apply "logic" which differs from that of the Supreme Court in Miller. The Supreme Court guidance was to hold a trial and acquit if the weapon was useful to a militia. But to convict, if it was not. This is equivalent to saying, it is not a crime for Miller and Layton to keep an arm useful to a militia. But it is a crime for Miller and Layton to keep a firearm that IS NOT useful to a militia.
The Second Amendment does not protect arms, it protects people, and according to the Miller Court it only protects people in keeping SOME arms. But inanimate objects do not have rights.
The Miller Court was completely comfortable with simply constraining the existing law in particular cases but not in all cases. But it very explicitly protected the right of Miller and Layton to keep an arm useful to a militia. A militia-membership test was argued by the prosecution but such requirement was not granted by the ruling.
Hmmmm. Personally, if the law was unconstitutional, I'd rather not have that on my record.
Better the judge dismisses the charges than to have my personal record say that I was arrested on a felony, jailed, tried and found not guilty.
But you're saying the Supreme Court suggested a trial and acquittal, huh?