Actually, short barrelled shotguns WERE used extensively during WWI, the relevent period for Miller. They were also used frequently during WWII, Korea and Vietnam. I even saw one guy carrying a sawed-off M-14... so a sawedoff 12 ga. IS, indeed, a useful militia/military weapon AND has the advantage that it's quite inexpensive (for the less well-heeled militia member) and can serve double duty as an ideal home defense weapon.
How short?