“In reality, Ragon was in favor of the gun control law and ruled the law unconstitutional because he knew that Miller, who was a known bank robber and had just testified against the rest of his gang in court, would have to go into hiding as soon as he was released. He knew that Miller would not pay a lawyer to argue the case at the Supreme Court and would simply disappear. Therefore, the government’s appeal to the Supreme Court would be a sure win because Miller and his attorney would not even be present at the argument....
...Gun rights advocates claim this case as a victory because they interpret it to state that ownership of weapons for efficiency or preservation of a well-regulated militia unit of the present day is specifically protected. Furthermore, such advocates frequently point out that short-barreled shotguns ( with 20 inch barrels) have been commonly used in warfare, and the statement made by the judges indicates that they were not made aware of this.
Because the defense did not appear, there was arguably no way for judges to know otherwise.
Two of the justices involved in the decision had prior military experience, Justice Black as a Captain in the field artillery during World War I and Justice Frankfurter as a Major in the Army legal service; however, there is no way to know if they were personally aware of the use of shotguns by American troops. During World War I, between 30,000 and 40,000 short-barreled pump-action shotguns were purchased by the US Ordnance Department and saw service in the trenches and for guarding German prisoners.
Some argue that fundamental issues related to the case were never truly decided because the Supreme Court remanded the case to the federal district court for “further proceedings” that never took place — by the time of the Supreme Court decision, Miller had been killed and Layton made a plea bargain after the decision was handed down, so there were no claimants left to continue legal proceedings...”
https://en.wikipedia.org/wiki/United_States_v._Miller
Odd case. Frankly, it was an embarrassment to the US to have a Supreme Court case where only one side showed up for the arguments...
My take is that the judge could not assume facts that were not presented into evidence before the court. Of course that would not be an issue for many modern SCOTUS judges.