Caliber:
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 hundredths or thousandths of an inch, or in millimeters. A bullet that is 0.451″ in diameter is 45 caliber.
The Cartridge Comparison Guide shown above is available on Amazon.