Your difficulty in answering that question belies the misguidedness thereof. The more people of your persuasion try to draw a line, the harder they find doing so.
Maybe there really isn't a line.
The Liberator pistol is, by your published reasoning, about as "unsuitable" as possible (very short smooth barrel, modest caliber, single shot, crummy quality, highly concealable, slow complicated reload, etc.) - yet a million were made in WWII precisely for "militia" (i.e.: anybody able & willing to fight in a declared war) purposes. Even a musket or sub-18" shotgun would be far preferable. Think about that: our government ordered production of a million of them specifically for our military allies.