As you noted, no one was present to argue for Miller in the Miller case. But any honest reading of the decision concludes that had it been demonstrated before the Court that a sawed off shotgun was indeed a regular military weapon that it would be protected under 2A.
Extrapolating from that so would automatic weapons, and even bazookas.
***no one was present to argue for Miller in the Miller case.***
From what I have read, the plaintiff had crossed the Arkansas line from Oklahoma to rob a bank in Siloam Springs AR when he was caught. Some say he was dead when this went to the SCOTUS.
https://encyclopediaofarkansas.net/entries/united-states-v-miller-et-al-4742/
hhttp://www.law.nyu.edu/sites/default/files/ECM_PRO_060964.pdf
The Letters of Marque and Reprisal section of the Constitution implies that privately owned major naval vessels are covered.