[PROCON]

Then why are the common artillery pieces 75, 105, 155mm etc referred to as such rather than by caliber?

155mm, 105mm, etc refers to diameter of the projectile.

Example: 155mm/25.4mm (1") = 6.1" diameter projectile.

[Bonemaker]

Navy referred to battleship cannons as 5, 14, 16 etc, inchers. Different nomenclature. Way back in the day they referred to artillery pieces in terms of “pounds”. Of course I’m not telling you anything you don’t know.