Calibre

noun

1. Diameter of the bore of a firearm, typically measured between opposite lands.
2. The diameter of round or cylindrical body, as of a bullet, a projectile, or a column.
3. A nominal name for a cartridge type, which may not exactly indicate its true size and may include other measurements such as cartridge length or black powder capacity. Eg 7.62×39 or 38.40.
4. Unit of measure used to express the length of the bore of a weapon. The number of calibres is determined by dividing the length of the bore of the weapon, from the breech face of the tube to the muzzle, by the diameter of its bore. A gun tube the bore of which is 40 feet (480 inches) long and 12 inches in diameter is said to be 40 calibers long.
5. Relative size, importance, magnitude.