| Gauge/Bore | |
|---|---|
 | |
| Type | firearms |
| Place of origin | Various |
Thegauge (inAmerican English; known asbore inCommonwealth English)[a] of afirearm is a unit of measurement used to express the inner diameter (bore diameter) and other necessary parameters to define in general asmoothborebarrel (compare tocaliber, which defines a barrel withrifling and itscartridge).
The gauge of ashotgun 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 validproof load[clarification needed], and commercial ammunition, as defined globally by theC.I.P.; defined in Great Britain by theRules, regulations and scales applicable to the proof of small arms (2006)[1] ofThe London Proof House andThe Birmingham Proof House, as referred in the Gun Barrel Proof Act 1978, Paragraph 6;[2] and defined in the United States bySAAMI Z299.2 – 2019.
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.[3]
The termgauge in connection of firearms was first used in the bookA Light to the Art of Gunnery (1677).[4]
Gauge was determined from the weight of a solid sphere of lead that would fit the bore of the firearm and is expressed as themultiplicative inverse of the sphere's mass as a fraction of a pound, e.g., a1⁄12 lb (38 g) 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 1 pound (453.6 g).[5] The term is related to the measurement ofcannons, which were also measured by the weight of their ironround shot; aneight-pounder would fire an 8-pound (3.6 kg) 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 adensity oflead inlbm⁄in3ρ, the formula for the bore diameter in inchesb based on the gaugeg was:
Due to problems defining apound, and to get pure lead, the Gun Barrel Proof Act 1855 defined a gauge as a list of defined values.[6] 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 hundredth of an inch.
Gauge is commonly used today in reference toshotguns, though historically it was first used inmuzzle-loadinglong guns such asmuskets, then later on inbreech-loadinglong guns includingsingle-shot anddouble rifles, which were made in sizes up to2 bore during their heyday in the mid to late 19th century, being originally loaded as black powder cartridges. These very large and heavy rifles, called "elephant guns", were intended for use primarily in regions of Africa and Asia for hunting large, dangerous game animals.
Gauge is commonly abbreviated as "ga.", "ga", or "G".
The British law that defined the relationship between bore diameter and gauge did so by listing every gauge from 1 to 50.[6] The list here does the same - the number of the list is the gauge, and the value to its right is the diameter in inches, as specified in the law. For convenience, the equivalent value in millimetres is provided, but this is not specified in the law.
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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 afraction of aninch, or as long as a few inches on some firearms. At the muzzle end of the barrel, thechoke 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 18.5 mm (0.73 in), can range from a tight 18 mm (0.71 in) to an extreme overbore of 20 mm (0.79 in). Some also claim an increased velocity with the overbored barrels, up to 15 m/s (49 ft/s), 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 thecenter of mass backward for better balance. Factory overbored barrels are generally made with a larger outside diameter and will not have this reduction in weight, although the factory barrels will be tougher, since they have a normal barrel wall thickness.
Firingslugs from overbored barrels can result in very inconsistent accuracy, as the slug may be incapable ofobturating to fill the oversized bore.
The six most common shotgun gauges, in descending order of size, are the 10 gauge,12 gauge, 16 gauge,20 gauge, 28 gauge, and.410 bore.[7] By far, the most popular is the 12 gauge,[7] particularly in the United States.[8] The 20-gauge shotgun is the next most popular size,[citation needed] and is popular forupland game hunting. The next most popular sizes are the .410 bore and the 28 gauge. The least popular sizes are the 10 gauge and the 16 gauge; while far less common than the other four gauges, they are still commercially available.[citation needed][9]
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 inmainland Europe. Today, they are rarely manufactured. These shells are usuallyblack powder paper or brass cartridges, as opposed to modernsmokeless powder plastic or wax cartridges.
The 18-, 15-, 11-, 6-, 3-, and 2-gauge shells are the rarest of all;[10] 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 the2+9⁄16-inch (65 mm) hull is still made in France.[10] The very small 24 and 32 gauges are still produced and used in some European and South American countries.Punt guns, which use very large shells, are rarely encountered.
Also seen in limited numbers aresmoothbore firearms in calibers smaller than .360 such as.22 long rifle (UK No. 1 bore) and9mm Flobertrimfire (UK No. 3 bore), designed for short-range pest control andgarden guns. The No. 2 bore (7 mm) has long been obsolete. All three of these rimfires are available in shot and BB-cap.[11][12]
The 10 gauge narrowly escaped obsolescence when steel and othernontoxic shot became required forwaterfowl 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. However, the3+1⁄2-inch (89 mm) 12-gauge shell, with its higherSAAMI pressure rating of 14,000 psi (97 MPa) compared to standard2+3⁄4-inch (70 mm) and 3-inch (76 mm) 12-gauge shells with their lower pressure rating of 11,500 psi (79 MPa), began to approach the performance of the3+1⁄2-inch (89 mm) 10-gauge shells with a pressure rating of 11,000 psi (76 MPa).[13] Newer nontoxic shots, such asbismuth ortungsten-nickel-iron alloys, and even tungsten-polymer blends, regain much or all of the performance loss, but are much more expensive thansteel orlead shot.[14]