A term that derives from Latin qua libra, meaning “what pound,” first applied to the weight of a bullet and then to the diameter. Caliber now refers to the diameter of either a projectile or the bore of a gun. It is the approximate bore or groove diameter expressed (in English) in hundredths of an inch. A bullet that is 0.451″ in diameter is 45 caliber. To write “.30 caliber” is technically incorrect. According to the strict definition, such a bullet would only be 0.003″ in diameter!