How do you measure caliber size of a rifle?

Answer 1

Bullets are usually categorised by calibre (nominal diameter) and cartridge case length. E.g. 7.62 x 51. (Millimetres).

Or, in some cases, it may be an Imperial measure, e.g. .50 (meaning .5 of an inch - 12.7 mm).

Further classification can indicate ammunition type (ball, AP, incendiary etc), user/ manufacturer (NATO for example).

Calibre is an approximation of the diameter of the bullet/ bore of the rifle.

Note that it's a nominal value only, for example the 7.62 bullet is actually 7.82 mm in diameter.