The gauge (in American English; known as bore in Commonwealth 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 gauge of a shotgun is a list that includes all necessary data to define a functional barrel. For example, the dimension of the chamber, the shotgun bore dimension, the valid proof load, and commercial ammunition, as defined globally by the C.I.P.; defined in Great Britain by the Rules, regulations and scales applicable to the proof of small arms (2006) of The London Proof House and The Birmingham Proof House, as referred in the Gun Barrel Proof Act 1978, Paragraph 6; and defined in the United States by SAAMI Z299.2 – 2019.
Historical development
thumb|200px|right|Portrait of [[Frederick Courteney Selous with his 4 bore single-shot Boer rifle and African hunting regalia, 1876]]
The concept of using a material property to define a bore diameter was used before the term "gauge" in the late 16th century.
The term gauge in connection of firearms was first used in the book A Light to the Art of Gunnery (1677).
Gauge was determined from the weight of a solid sphere of lead that would fit the bore of the firearm and is expressed as the multiplicative inverse of the sphere's mass as a fraction of a pound, e.g., a 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 mass . The term is related to the measurement of cannons, which were also measured by the weight of their iron round shot; an eight-pounder would fire an ball. Therefore, a 12-gauge is larger than a 16-gauge. Since the term was defined in terms of pounds and inches, this implied that for a density of lead in , the formula for the bore diameter in inches based on the gauge was:
<math display=block>\begin{align}
b &= 2\sqrt[3]{\frac{3}{4\pi g\rho &&\approx 2\sqrt[3]{\frac{3}{1.64\pi g &&\approx 1.67\sqrt[3]{\frac{1}{g\\
\rho &\approx 0.41 \frac{\mathrm{lb}_{\mathrm{m}{\mathrm{in}^3}
\end{align}
</math>
Due to problems defining a pound and getting pure lead, the Gun Barrel Proof Act of 1855 defined a gauge as a list of defined values. As a result, the formula above only works as a heuristic for determining bore diameter from gauge, as the table in the law lists diameters not consistent with any possible density, even after accounting for rounding errors inherent in the table only being precise to a thousandth of an inch.
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, originally loaded as black powder cartridges. These very large and heavy rifles, called "elephant guns", were intended primarily for use in regions of Africa and Asia to hunt large, dangerous game animals.
Gauge is commonly abbreviated as "ga.", "ga", or "G".
Legal Definition
The British law that defined the relationship between bore diameter and gauge listed every gauge from 1 to 50. By far, the most popular is the 12 gauge,
Shotguns and shells exceeding 10 gauge, such as the 8 gauge, 6 gauge, 4 gauge, and 2 gauge, are historically important in the United Kingdom and elsewhere in mainland Europe. Today, they are almost exclusively made by custom order or handloaders. These shells are usually black powder paper or brass cartridges, as opposed to modern smokeless powder plastic or wax cartridges.
The 18-, 15-, 11-, 6-, 3-, and 2-gauge shells are the rarest of all; owners of these types of rare shotguns will usually have their ammunition custom-loaded by a specialist in rare and custom bores. The 14 gauge has not been loaded in the United States since the early 20th century, although the hull is still made in France.
Gauge and shot type
thumb|right|A 10-gauge () shotgun shell shown next to a [[Quarter (United States coin)|United States quarter]]
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-gauge shells for waterfowl hunting, and the shorter, , 12-gauge shells as well. However, the 12-gauge shell, with its higher SAAMI pressure rating of compared to standard and 12-gauge shells with their lower pressure rating of , began to approach the performance of the 10-gauge shells with a pressure rating of . Newer nontoxic shots, such as bismuth or tungsten-nickel-iron alloys, and even tungsten-polymer blends, regain much or all of the performance loss, but are much more expensive than steel or lead shot.