Doesn’t it simplify as projectile versus EXPLOSIVE shell?
Mostly .... though it would be easy enough to create a couple of quick rules like you mentioned.
Ordnance:
Crew served weapon (takes 2 or more to operate)
Fires an explosive projectile
Fires a non-explosive projectile greater than 3/4 inch in diameter
Arms (thus qualifying for 2nd Amendment protections):
designed to be operated by an individual
fires a non-explosive projectile equal to or less than a 3/4 inch in diameter (covers most shotguns and black powder rifles).