See the link to the research on the Miller case in comment #1. It turns out the trial court judge was a buddy of the Roosevelt administration and extremely anti-second amendment. He structured the case the way it was on purpose, so that the administration could get the ruling it wanted without any evidence from the defense.
This was judicial activism at its worst.
It turns out the trial court judge was a buddy of the Roosevelt administration and extremely anti-second amendment. Interesting: he needed to have the Supreme Court hear the case before Miller et al. could present evidence.
BTW, reading more of the history, it seems even clearer why Miller needed the shotgun; by stealing it, the government is complicit in Miller's murder.