"And if could not be shown to be a common part?"
Absent more unchallenged lies on the part of the persecutor, there is no possible WAY it could not have been EASILY shown. Experience from the Great War (WWI) in the trenches, where sawed-off shotguns were pretty prevalent, would have easily shown the military untility of Miller's weapon. Hell's fire, they were very big in WWII in the Pacific and we used them in Vietnam, as well. I even saw one guy carrying a sawed-off M-14 rifle. A sawed off 12ga. is absolutely the ideal weapon for close-in work, bar none. It becomes useless after more than 15 yards or so, because the shot spreads too much, but under that range, you are virtually guaranteed a rather messy kill.
Anything is "useful" to a militia. But the question was whether Miller's weapon was one that would be used by average Militia member to take into combat and be effective.
I don't think his weapon would have qualified. In my opinion. We'll never know.
Years ago I sawed off a beat up old Sears boxlock dbl to 26" and patterned it at 40 yds with #4 buck.. Almost all pellets were within a 5' circle.
I then cut it down to 22", and it put nearly all of its 21 pellets in an approx 6' circle.
Finally, cut down to a bit over 18" the gun still put [consistently] 90% of its 21 pellet #4 buck load into a 72" circle at 40 yards.
-- A standing man at the center of that circle would get hit by at least 4 to 6 .24 caliber pellets, still moving at 800FPS or so..
Never sell buckshot 'short', -- it's still lethal at 40 yards.