At the time of Miller, yes, short barreled shotguns had been used by the military. Just not that short.
"the Court would have found that Miller had a right to possess it."
Mr. Miller was not arrested for possessing a short barreled shotgun. He was arrested for possessing a short barreled shotgun without a federal tax stamp.
You can argue all you want whether of not this was a suitable militia weapon. The fact is, it lacked a stamp. He violated the federal law that required one on this type of weapon.
Where’s your tax stamp for that Internet post?
And the lower court found that law violated Millers rights, no? And reading the Miller court makes it fairly clear that had anyone been present to argue Millers case, the SC would have upheld the lower court.