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 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 A sphere () is a Geometry, geometrical object that is a solid geometry, three-dimensional analogue to a two-dimensional circle. A sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...

lead Lead is a chemical element with the Symbol (chemistry), symbol Pb (from the Latin ) and atomic number 82. It is a heavy metals, heavy metal that is density, denser than most common materials. Lead is Mohs scale of mineral hardness#Intermediate ...

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''/''b ...

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 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 duri ...

, 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 la ...

; 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-barreled firearm designed to shoot a straight-walled cartridge known as a shotshell, which usually discharges numerous small pellet-like spherical sub- p ...

s, 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, then ...

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 ...

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/cm³ or 0.4097 lb/in³) with that diameter has a mass equal to part of the mass of the

international avoirdupois pound The pound or pound-mass is a unit of mass used in British imperial and United States customary systems of measurement. Various definitions have been used; the most common today is the international avoirdupois pound, which is legally define ...

(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 :

d_n = 2 \sqrt /math>

Explanation:

* Divide 1 pound by ''n'' to find the mass of each one of the balls
* Divide it by 0.4097 lb/in

³ (density of lead) to find the volume of each ball * Multiply it by and divide it by pi, then find its cube root (rearranged from the

volume of a 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 the ...

equation) to find the radius of each ball * Multiply it by 2 to change from radius to diameter This simplifies to the following formula for the internal diameter of the barrel of an ''n''-gauge shotgun: :

d_n=1.67/\sqrt

(in inches), or

d_n=42.4/\sqrt

(in millimeters). Likewise, given the diameter in inches, the gauge is :

n=4.66/d_n^3

The gauge of firearms is determined by: 1 pound/gauge = weight of lead sphere. Caliber of the bore is then measured. This is essential information for understanding gauges.

Bore sizing

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. The fact that most shotgun bores are not cylindrical also causes deviations from the ideal bore diameter. The chamber of the gun is larger, to accommodate the thickness of the shotshell walls, and a "forcing cone" in front of the chamber reduces the diameter down to the bore diameter. The forcing cone can be as short as a fraction of an inch, or as long as a few inches on some guns. At the muzzle end of the barrel, the choke can constrict the bore even further, so measuring the bore diameter of a shotgun is not a simple process, as it must be done away from either end. Shotgun bores are commonly "overbored" or "backbored", meaning that most of the bore (from the forcing cone to the choke) is slightly larger than the value given by the formula. This is claimed to reduce felt recoil and improve patterning. The recoil reduction is due to the larger bore producing a slower acceleration of the shot, and the patterning improvements are due to the larger muzzle diameter for the same choke constriction, which results in less shot deformation. A 12-gauge shotgun, nominally , can range from a tight to an extreme overbore of . Some also claim an increased velocity with the overbored barrels, up to , which is due to the larger swept volume of the overbored barrel. Once only found in expensive custom shotguns, overbored barrels are now becoming common in mass-marketed guns. Aftermarket backboring is also commonly done to reduce the weight of the barrel, and move the center of mass backward for a better balance. Factory overbored barrels generally are made with a larger outside diameter, and will not have this reduction in weight—though the factory barrels will be tougher, since they have a normal barrel wall thickness. Firing

slugs Slug, or land slug, is a common name for any apparently shell-less terrestrial gastropod mollusc. The word ''slug'' is also often used as part of the common name of any gastropod mollusc that has no shell, a very reduced shell, or only a sm ...

from overbored barrels can result in very inconsistent accuracy, as the slug may be incapable of obturating to fill the oversized bore.

Gauges in use

The six most common shotgun gauges, in descending order of size, are the 10 gauge, 12 gauge, 16 gauge,

20 gauge The 20- gauge shotgun, also known as "20-bore", is a type of smooth-bore shotgun that fires a shell that is smaller in caliber () than a 12-gauge shotgun (). It is often used by beginning shooters for target practice and for hunting small game. ...

, 28 gauge, and

.410 bore The .410 bore is one of the smallest caliber of shotgun shell commonly available (along with the 9mm Flobert rimfire cartridge, and the less common .22 rimfire shot shell). A .410 bore shotgun loaded with shot shells is well suited for small g ...

. By far the most popular is the 12 gauge, particularly in the United States. The 20-gauge shotgun is the next most popular size being favored by shooters uncomfortable with the weight and recoil of a 12-gauge gun, and is popular for upland game hunting. The next most popular sizes are 28 gauge and .410 bore. Both the 10 gauge and 16 gauge, while less commonplace, are still available. Shotguns and shells exceeding 10 gauge, such as the 8, 6, and 4 gauge, are historically important in the United Kingdom and elsewhere in mainland Europe. Today, they are rarely manufactured. Shells are usually black powder paper cartridges as opposed to the plastic or wax cartridge and smokeless powder of today. 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 1900s, although the hull is still made in France. The very small 24 and 32 gauges are still produced and used in some European and South American countries.

Punt gun A punt gun is a type of extremely large shotgun used in the 19th and early 20th centuries for shooting large numbers of waterfowl for commercial harvesting operations. These weapons are characteristically too large for an individual to fire from ...

s are rarely encountered. Munit05

Also seen in limited numbers are

smoothbore A smoothbore weapon is one that has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars. History Early firearms had smoothly bored barrels that fired projectiles without s ...

firearms in calibers smaller than .360 such as

.22 Long Rifle The .22 Long Rifle or simply .22 LR or 22 (metric designation: 5.6×15mmR) is a long-established variety of .22 caliber rimfire ammunition originating from the United States. It is used in a wide range of rifles, pistols, revolvers, smo ...

(UK No. 1 bore) and 9mm Flobert rimfire (UK No. 3 bore), designed for short-range pest control and garden guns. The No. 2 bore (7 mm) has long been obsolete. All three of these rimfires are available in shot and BB-cap.

Gauge and shot type

The 10 gauge narrowly escaped obsolescence when steel and other nontoxic shot became required for

waterfowl hunting Waterfowl hunting (also called wildfowling or waterfowl shooting in the UK) is the practice of hunting ducks, geese, or other waterfowl for food and sport. Many types of ducks and geese share the same habitat, have overlapping or identical h ...

, 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 smaller, , 12-gauge shells as well. However, the 12-gauge shell, with its higher

SAAMI The Sporting Arms and Ammunition Manufacturers' Institute (SAAMI, pronounced "Sammy") is an association of American manufacturers of firearms, ammunition, and components. SAAMI is an accredited standards developer that publishes several Ame ...

pressure rating compared to standard 12-gauge guns, began to approach the performance of the 10-gauge loads. Newer nontoxic shots, such as

bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...

and

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

-nickel-iron alloys, and even tungsten-

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

blends, regain much or all of the performance loss, but are much more expensive than steel or lead shot. However, laboratory research indicates that tungsten alloys can actually be quite toxic internally.

Sizes found in the United Kingdom

Legend: left size is bore size, right side is case length. *

4 bore Four bore or 4 bore is a mostly obsolete black powder caliber of the 19th century, used for the hunting of large and potentially dangerous game animals. The specifications place this caliber between the larger two bore and the lesser six bore. ...

: *

8 bore The 8 bore, also known as the 8 gauge, is an obsolete caliber used commonly in the 19th-century black-powder firearms. Design An 8 bore is a caliber firearm. Historically it was used to fire solid projectiles from smoothbores, rifles, and part ...

: *10 bore: *12 bore: *14 bore: *16 bore: *20 bore: *24 bore: *28 bore: *32 bore: *

: *.360 bore: *9 mm (No. 3 bore) rimfire

.5 in One half ( : halves) is the irreducible fraction resulting from dividing one by two or the fraction resulting from dividing any number by its double. Multiplication by one half is equivalent to division by two, or "halving"; conversely ...

9 mm (No. 3 bore) long rimfire

.4 in 0.4, .4, point four, or dot four may refer to: * The number 0.4 as a fractional number * 0.4 shot, a basketball shot made by player Derek Fisher with 0.4 seconds left in Game 5 of the NBA 2004 Western Conference Semifinals * Point4, a songwriting ...

*7 mm (No. 2 bore) rimfire *6 mm (No. 1 bore) short rimfire, 6 mm (No. 1 bore) long rimfire

Conversion guide

The table below lists various gauge sizes with weights. The bores marked ^* are found in punt guns and rare weapons only. However, 4 gauge were sometimes found used in

blunderbuss The blunderbuss is a firearm with a short, large caliber barrel which is flared at the muzzle and frequently throughout the entire bore, and used with shot and other projectiles of relevant quantity or caliber. The blunderbuss is commonly con ...

guns made for coach defense and protection against piracy. The .410 and 23 mm are exceptions; they are actual bore sizes, not gauges. If the .410 and 23 mm were measured traditionally, they would be 67.62 gauge and 6.278 gauge, respectively. Note: Use of this table for estimating bullet masses for historical large-bore rifles is limited, as this table assumes the use of round ball, rather than conical bullets; for example, a typical 4-bore rifle from circa 1880 used a bullet, or sometimes slightly heavier, rather than using a round lead ball. (Round balls give progressively much worse external ballistic performance than conical bullets at ranges greater than about ) In contrast, a 4-bore

express rifle The term ''express'' was first applied to hunting rifles and ammunition beginning in the mid-19th century, to indicate a rifle or ammunition capable of higher than typical velocities. The early express cartridges used a heavy charge of black po ...

often used a bullet wrapped in paper to keep lead buildup to a minimum in the barrel. In either case, assuming a 4-ounce mass for a 4-bore rifle bullet from this table would be inaccurate, although indicative.

References

{{DEFAULTSORT:Gauge (Bore Diameter) Shotgun shells Units of length