The US Prosecutor essentially lied to the USSC justices and since Mr. Miller or his co-defendant Mr. Stevens could not afford to make the trip nor did he hire counsel, there was no one there to refute whatever the prosecutor said. So he lied. The justices never questioned his arguments. The NFA was upheld.
From Unintended Consequences:
The Second Amendment is a recognition of the danger of standing armies. Its purpose is to recognize that every citizen has the right to keep and bear the same type of basic arms as a soldier in a modern military.
A militia embodies all able-bodied men over the age of sixteen. Therefore, a militia will always outnumber a standing army by at least twenty to one. If this militia is armed with weapons similar to those used by the individuals comprising the standing army, it will be impossible for that standing army to inflict the will of a tyrannical government upon the people.
The Second Amendment is the guarantee behind all the other articles in the Bill of Rights. It is the ultimate guarantee that citizens in the United States will remain free.
SCOTUS said that it lacked the information it needed to decide the case, BUT IF the weapon in question "has some reasonable relationship to the preservation or efficiency of a well regulated militia" or "is any part of the ordinary military equipment, or that its use could contribute to the common defense", then that weapon is in the ambit of protection of the 2nd amendment. So, if a shot barrel shotgun fits within one of those categories, then that part of the NFA was ruled to be unconstitutional.
Miller was not only a no-sow at SCOTUS, he was also unavailable on remand, so a judicial finding was never produced, whether or not a shotgun with a barrel of less than 18 inches has some military use.
I still blame SCOTUS, because its Miller holding was butchered chronically, and appeals taken to SCOTUS, which denied cert.
When it comes to the 2nd amendment, the federal court system is an open scam.