Shotgun barrels are classified by "gauge"--a term that refers indirectly to the bore diameter of a shotgun. It is not a direct measurement but is calculated as the number of lead balls of a bore diameter required to make up one pound. This is an old British measurement system. Gauge is determined by taking lead balls the same diameter of the gun's bore, then counting the total number of those lead balls it takes to equal one pound. For example: a 12-gauge= 12 lead balls to the pound; a 20-gauge=20 lead balls to the pound.