The gauge (in American English or more commonly referred to as bore in British English) of a firearm is a unit of measurement used to express the inner diameter (bore diameter) and other necessary parameters to define in general a smoothbore barrel (compare to caliber, which defines a barrel with rifling and its cartridge).

The concept of using a material property to define a bore diameter was used before the term gauge, in the end of the 16th century.

[4] Gauge was 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 a pound, e.g., a one-twelfth pound lead ball fits a 12-gauge bore.

Therefore with a 12-gauge, it would take 12 balls of lead of the same size as the 12 gauge shotgun's inner bore diameter to weigh 1 pound (453.6 g).

[6] Gauge is commonly used today in reference to shotguns, though historically it was first used in muzzle-loading long guns such as muskets, then later on in breech-loading long guns including single-shot and double rifles, which were made in sizes up to 2 bore during their heyday in the mid to late 19th century, being originally loaded as black powder cartridges.

Since shotguns were not originally intended to fire solid projectiles, but rather a compressible mass of shot, the actual diameter of the bore can vary.

Aftermarket backboring is also commonly done to reduce the weight of the barrel and move the center of mass backward for a better balance.

The 14 gauge has not been loaded in the United States since the early 20th century, although the 2+9⁄16-inch (65 mm) hull is still made in France.

[11][12] The 10 gauge narrowly escaped obsolescence when steel and other nontoxic shot became required for waterfowl hunting, since the larger shell could hold the much larger sizes of low-density steel shot needed to reach the ranges necessary for waterfowl hunting.

The move to steel shot reduced the use of 16 and 20 gauges for waterfowl hunting, and the shorter, 2+3⁄4-inch (70 mm), 12-gauge shells as well.

(Round balls lose velocity faster than conical bullets and have much steeper ballistic trajectories beyond about 75 yards or 69 metres) In contrast, a 4-bore express rifle often used a 1,500-grain (3.43 oz; 97.20 g) bullet wrapped in paper to keep lead buildup to a minimum in the barrel.