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.
How many of those million were used by our miltary during WWII? How many of those are currently being used by our military?
Yet you consider those weapons to bear some reasonable relationship to the preservation or efficiency of a well regulated militia? Do you know the difference between that and "this thing can kill people"?
I honestly don't think you do.