The gauge (or commonly bore in British English) of a
firearm is a unit of measurement used to express the inner diameter (bore diameter) of the
barrel.
Gauge is determined from the weight of a solid
sphere
A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is th ...
of
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
that will fit the bore of the firearm and is expressed as the
multiplicative inverse
In mathematics, a multiplicative inverse or reciprocal for a number ''x'', denoted by 1/''x'' or ''x''−1, is a number which when multiplied by ''x'' yields the multiplicative identity, 1. The multiplicative inverse of a fraction ''a''/ ...
of the sphere's weight as a fraction of a
pound, e.g., a one-twelfth pound lead ball fits a 12-gauge bore. Thus there are twelve 12-gauge balls per pound, etc. The term is related to the measurement of
cannon
A cannon is a large- caliber gun classified as a type of artillery, which usually launches a projectile using explosive chemical propellant. Gunpowder ("black powder") was the primary propellant before the invention of smokeless powder ...
, which were also measured by the weight of their iron
round shot
A round shot (also called solid shot or simply ball) is a solid spherical projectile without explosive charge, launched from a gun. Its diameter is slightly less than the bore of the barrel from which it is shot. A round shot fired from a lar ...
; 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 bore Two bore or 2 bore is a mostly obsolete firearm caliber.
Specifications
Two bores generally fire spherical balls or slugs of hardened lead or, in the modern metallic cartridge, additionally a solid bronze projectile. The nominal bore is , and pr ...
during their heyday in the 1880s, being originally loaded with black powder cartridges. These very large rifles, called "
elephant gun
An elephant gun is a large caliber gun, rifled or smoothbore, originally developed for use by big-game hunters for elephant and other large game. Elephant guns were black powder muzzle-loaders at first, then black powder express rifles, the ...
s", were intended for use primarily in
Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
and
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...
for hunting large dangerous game.
Gauge is abbreviated "ga.", "ga", or "G".
Calculating gauge
An ''n''-gauge diameter means that a ball of lead (density 11.34 g/cm
3 or 0.4097 lb/in
3) with that diameter has a mass equal to part of the mass of the
international avoirdupois pound (approx. 454 grams), that is, that ''n'' such lead balls could be cast from a pound weight of lead. Therefore, an ''n''-gauge shotgun or ''n''-bore rifle has a bore diameter (in inches) of approximately
: