Gauge
From left to right; a.45 ACP cartridge, a.410 bore shotshell, a20-gauge shotshell, and a 12-gauge shotshell
Type	Shotgun
Place of origin	Various

Thegauge (in American English or more commonly referred to asbore in British English) 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 and the valid proof load 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 term gauge in connection of firearms was first used in the book,A Light to the Art of Gunnery (1677).^[4]

Gauge was determined from the weight of a solid sphere of lead that will fit the bore of the firearm and is expressed as themultiplicative inverse of the sphere's weight as a fraction of a pound, e.g., a one-twelfth pound 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 weigh 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.

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]

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".

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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 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.

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] However, laboratory research indicates that tungsten alloys can actually be quite toxic internally.^[15]

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Legend: left side is the bore size, right side is the case length

• 2 bore:3+1⁄2 in (89 mm)
• 4 bore:3+1⁄4, 4 in (83, 102 mm)
• 6 bore
• 8 bore:3, 3+1⁄4, 3+3⁄4, 4 in (76, 83, 95, 102 mm)
• 10 bore:2+9⁄16, 2+5⁄8, 2+7⁄8, 3, 3+1⁄4, 3+1⁄2 in (65, 67, 73, 76, 83, 89 mm)
• 12 bore:1+3⁄4, 2, 2+1⁄4, 2+1⁄2, 2+3⁄4, 3, 3+1⁄2 in (44, 51, 57, 64, 70, 76, 89 mm)
• 14 bore:2+1⁄2 in (64 mm)
• 16 bore:2+1⁄2, 2+9⁄16, 2+5⁄8, 2+3⁄4, 3 in (64, 65, 67, 70, 76 mm)
• 20 bore:2+1⁄2, 2+3⁄4, 3, 3+1⁄2 in (64, 70, 76, 89 mm)
• 24 bore:2+1⁄2 in (64 mm)
• 28 bore:2+1⁄2, 2+3⁄4, 3 in (64, 70, 76 mm)
• 32 bore:2+1⁄2 in (64 mm)
• .410 bore:2, 2+1⁄2, 3 in (51, 64, 76 mm)
• .360 bore:1+3⁄4 in (44 mm)
• 9 mm (No. 3 bore) short rimfire, 9 mm (No. 3 bore) long rimfire
• 7 mm (No. 2 bore) rimfire
• 6 mm (No. 1 bore) short rimfire, 6 mm (No. 1 bore) long rimfire

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The table below lists various gauge sizes with weights. The bores marked^* are found in punt guns, obsolete, or rare weapons only. However, 4 gauge was sometimes found used inblunderbuss guns made for coach defense and protection against piracy. The.410 and23 mm are exceptions; they are actual bore sizes, not gauges. If the .410 bore and 23 mm diameters were measured using more traditional means, they would be equivalent to 67.62 gauge (.410 bore) and 6.278 gauge (23 mm), 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 2,000-grain (4.57 oz; 129.60 g) bullet, or sometimes slightly heavier, rather than using a 4-ounce (110 g) round lead ball. (Round balls lose velocity faster than conical bullets and have much steeper ballistic trajectories beyond about 75 yards or 69 metres) In contrast, a 4-boreexpress rifle often used a 1,500-grain (3.43 oz; 97.20 g) bullet wrapped in paper to keep lead buildup to a minimum in the barrel. In either case, assuming a 4-ounce (110 g) mass for a 4-bore rifle bullet from this table would be inaccurate, although indicative.

• Shotgun cartridges
• Units of length

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