This specific measurement refers to the cartridge case used in a popular intermediate rifle cartridge. The first number indicates the bullet diameter, the second the case length. These precise measurements are critical for proper chambering and functioning in firearms designed for this ammunition.
Standardization of cartridge dimensions ensures reliability and interchangeability. Variations, even minor ones, can affect chamber pressure, accuracy, and even firearm safety. The historical development of these specifications reflects the ongoing evolution of firearms technology and the pursuit of balanced performance characteristics, including projectile weight, velocity, and recoil.