20 Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm and is expressed as the multiplicative inverse of the sphere’s weight as a fraction of 20 GAUGE pounds, e.g., a one-twelfth pound lead ball fits a 12-gauge bore. Thus there are twelve per pound, etc. The term is related to the measurement of cannon, which were also measured by the weight of their iron round shot; an 8-pounder would fire an 8 lb (3.6 kg) ball.
Gauge is commonly used today in reference to shotguns, though historically it was also used in large double rifles, which were made in sizes up to 2 boress during their heyday in the 1880s, being originally loaded with black powder cartridges. These very large rifles, called “elephant guns“, were intended for use in Africa and Asia for hunting games.