What's seldom mentioned is the bottom line of U.S. v. Miller: the case was REMANDED to trial court. Few people seem to grasp the significance of this.
Trial court is generally the only place where one may introduce controversial evidence may be used in making a final decision (generally, evidence introduced in other ways is only usable in deciding whether a case should go to, or return to, trial court). The indictment against Miller was initially thrown out before his case ever got to trial court, and thus before he had any opportunity to present any evidence of anything. The Supreme Court didn't find evidence that a sawed-off shotgun wasn't a militarily-suitable weapon, nor did it find that since Jack Miller had failed to present in timely fashion evidence of military suitability, it could affirmatively rule that he could not make such a claim (having never gone to trial, Miller had not yet had the opportunity to present such evidence). All it found was that the case couldn't be thrown out on Second-Amendment grounds until the question of military utility had been addressed in trial court.
Had the case against Miller and Layton gone to trial court, the two of them would have had the opportunity to present evidence that short shotguns were sometimes used in World War I. The government would have had a very time winning its case; although trial courts do not set legally-binding precedents, if it became well-known that Miller was acquitted despite clear evidence that he had possessed the weapons in question without proper tax payment, NFA offenses would have become unprosecutable.
I wish an anti-gun legal scholar could explain a couple things:
This was judicial activism at its worst.
I appreciate your posting to the effect that the Miller Court did NOT decide whether the keeping and bearing of a short-barreled shotgun was protected by the Second Amendment.
Where I think your statement is incorrect is in shifting the burden of proof to the defendants. The burden of showing that the defendants were not protected by the Second Amendment should fall on the prosecution not the defense. It should have been the prosecution's burden to prove that the short-barreled shotgun WAS NOT useful to a Militia.
Anything else would be transforming the protections of the Bill of Rights into "affirmative defenses" with an unknowable burden on the defense.
This issue is why I believe that the Miller decision is fatally flawed. It presumes that the Second Amendment only applies to "some arms", which is a limitation that the Founders could have included. There is no evidence that self-defense was to be excluded, as is now the topic of the Heller case. If self-defense is recognized, then Miller disappears as a useful precedent and the decision of the original trial court to dismiss Miller's case is seen for the correct decision that it was.