‘Howitzers, for instance, are not protected. Nor are .50 caliber machine guns, among others. ‘
Disagree! Theres nothing in the 2nd that says or implies this. In fact it was common for private individuals to own arty pieces during our early history. The only restriction was the depth or your pockets.
Swivel guns were commonly mounted on the rails of ships to repel boarding parties. Potter's was reportedly loaded with rifle-ball-sized shot, (making it a huge "shotgun") and it was intended as the last line of drfense if hostile Indians breached the door.
At the time, such private defense artillery was not uncommon -- even for folks who were not public figures.
So, the idea of the right to "privately-owned artillery" extended well beyond revolutionary days.