The gauge (or commonly bore in British English) of a
firearm
A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see Legal definitions).
The first firearms originated in 10th-century China, when bamboo tubes ...
is a unit of measurement used to express the inner diameter (bore diameter) of the
barrel
A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids, ...
.
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 a
pound
Pound or Pounds may refer to:
Units
* Pound (currency), a unit of currency
* Pound sterling, the official currency of the United Kingdom
* Pound (mass), a unit of mass
* Pound (force), a unit of force
* Rail pound, in rail profile
Symbols
* Po ...
, 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, 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
shotgun
A shotgun (also known as a scattergun, or historically as a fowling piece) is a long gun, long-barreled firearm designed to shoot a straight-walled cartridge (firearms), cartridge known as a shotshell, which usually discharges numerous small p ...
s, though historically it was also used in large
double rifle
The double rifle, also known as a double-barreled rifle, is a rifle with two barrels mounted parallel to each other. Synonymous with big game hunting found primarily in Africa and India, the double rifle is a purely sporting weapon with no militar ...
s, 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 guns", were intended for use primarily in
Africa and
Asia 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
: