Why are naval guns measured in inches and Army guns measured in millimeters?

Answer 1

Because large naval rifles are built to fire at ranges out to as much as 18 miles or more, they do not fire fixed ammunition. The powder charge for a given projectile weight, range and elevation is calculated by the number of standard powder bags. In order to do the calculations, the volume of the barrel must be known, because the powder burns behind the projectile during its entire trip toward the muzzle of the gun. Too little powder, and power is lost; too much, and the gun might blow up.

Thus, the "caliber" of naval guns is stated as (for example) "10-inch, 45 caliber," with the inches being the diameter of the barrel and "45 caliber" meaning that the barrel length is 45 times its diameter, or 450 inches. This describes the size of the projectile and the volume of the bore.

In fixed ammunition, where the powder charge is contained in a cartridge, these calculations are not necessary. Caliber in modern field guns has come to mean the diameter of the barrel. As to metric measurement, it is easier to say "120 mm" than to say 4.72 inches, and it standardizes sizes with the rest of the world.