For instance on a US Naval destroyer that has a 5" deck gun it is a .54 caliber round that is projected from the muzzle. They even have a new Mk45 .63 caliber extended range model. It has same projectile diameter yet extends barrel length a few inches which changes the caliber.
Now why is a 5 inch round only measured to 54 caliber? I'll let you fellow sailors do the math on that one.