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Gauge (bore diameter): Wikis


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Shotshell 001.jpg
From left to right; a .45 ACP, a .410 bore Shotshell, a 20 gauge shotshell, and a 12 gauge shotshell
Type Shotgun
Place of origin Various

The gauge of a firearm is a unit of measurement used to express the diameter of the barrel. 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 (e.g. a 1/12th pound ball fits a 12-gauge bore). 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 pound (3.6 kg) spherical cast iron ball and had a bore diameter of about 91 mm (3.6 inches).

Commonly used today in reference to shotguns, though historically it was also used in large double rifles, which were made in sizes up to 4 bore during their heyday in the 1880s, being originally loaded with black powder cartridges. These very large rifles, sometimes called elephant guns, were intended for use in India and Africa for hunting dangerous game.


Calculating gauge

An n-gauge diameter means that a ball of lead (density 11.352 g/cm3 or 6.562 oz/cu in) with that diameter has a mass equal to 1/n part of the mass of the international avoirdupois pound (453.59237 grams). Therefore an n-gauge shotgun or n-bore rifle has a bore diameter (in centimeters) of approximately

d_n = \left(\frac{6 \times 453.59237~\mathrm{g}}{11.352~\mathrm{g/cm}^3 \times n \times \pi}\right)^{1/3} = 4.2416~\mathrm{cm} \times \frac{1}{\sqrt[3]{n}}

Another source for a gauge size formula can be found at shotgun shell.

Gauges in use

From left to right: 9x19mm Parabellum, .40 S&W, .45 ACP, 5.7x28 mm, 5.56 x 45 mm NATO, .300 Winchester Magnum, and a 2.75-inch and 3-inch 12 gauge

Since shotguns were not originally intended to fire solid projectiles, but rather a compressible mass of shot, the actual diameter of the bore varies quite a bit. 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 four 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 18.5 mm (0.730 inches), can range from a tight 18.3 mm (0.720 in) to an extreme overbore of 20.3 mm (0.800 in). Some also claim an increased velocity with the overbored barrels, up to 15 m/s (50 feet per second), 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 backwards 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 from overbored barrels can result in very inconsistent accuracy, as the slug may be incapable of obturating to fill the oversized bore.

Sizes in use

Certain sizes are more common than others; 12 gauge is the most common size[citation needed], with up to 50% of the overall shotgun market in the United States. The 20-gauge shotgun is popular with shooters who are uncomfortable with the weight and recoil of a 12 gauge gun, and is popular for upland game hunting. The next most popular size is the .410 shotgun, which is not a gauge but a caliber. 10, 16, and 28 gauges, while less common, are still readily available.

Shotguns larger than 10 gauge are rarely manufactured nowadays[citation needed]. 8 gauge is rare in the United States due to its prohibition in duck hunting. However, it is still used in many parts of the world (notably: Britain) for bird hunting. Its shells are usually black powder paper cartridges as opposed to the plastic/wax cartridge and smokeless powder of today.

14, 18, and 11 gauge rounds are the rarest of all the shells[citation needed]; people that own these types of rare shotguns will usually have their ammunition custom reloaded by a company that specializes in rare and custom bores for a high price. 24 and 32 gauges are still produced and used in some European countries and Brazil. Punt guns and special purpose guns like the Russian 23 mm KS-23 (approximately 6 gauge) do exist, but are rarely encountered.

Also seen in limited numbers are smoothbore firearms in calibers smaller than .410, such as .22 Long Rifle, .22 WMR, and 9 mm rimfire, designed for short range pest control.[1][2]

To further complicate matters, special shot cartridges are available for typical handgun chamberings such as 9 mm Parabellum, .45 ACP, .38 Special/.357 Magnum, .44 Special/.44 Magnum, and .45 Colt. These rounds are either crimped in or in a plastic casing replacing the bullet. These are not generally considered "shot shells" by shotgun users, and the patterning performance is questionable since they are fired through rifled barrels. Thompson/Center makes special pistol barrels in .38/.357, .44 and .45 Colt that have "straight rifled" chokes in them to reduce the spin of the shot column and produce better patterns, but they are still suitable only for pest control at very short ranges.


Gauge and shot type

The 10 gauge was headed into obsolescence until steel and other non-toxic shot started to be required for waterfowl hunting, as the larger case could hold the much larger sizes of shot needed to get the low density steel shot to the ranges needed for waterfowl hunting; this same move effectively shut out the 20 and smaller gauges as well. However, the advent of the 3 1/2" (89 mm) 12 gauge shell, with its higher SAAMI pressure rating, meets or exceeds the performance of the 10 gauge loadings. Newer non-toxic shots, such as bismuth and tungsten-nickel-iron alloys, and even tungsten-polymer blends, regain much or all of the performance loss, but at much higher cost than steel or lead shot.[3] 10 gauge is capable of 3 oz of shot @ 1200fps far more powerful than a 12 gauge 3.5

Conversion guide

Portrait of Frederick Courteney Selous with his 4 bore single-shot Boer rifle and African hunting regalia, 1876.

A table showing the various gauge sizes with weights. The bores marked * are found in punt guns and rare weapons only. The .410 bore is an exception; it is an actual bore size, not a gauge. If the .410 were measured traditionally, it would be 67½ gauge. The European 4-gauge (24 mm, nominal, caliber) is sized slightly different than the traditional 4-bore or 4-gauge, being based on the newer metric 'CIP' tables.

Caliber Weight of unalloyed (pure) lead ball
(mm) (in) (g) (oz) (gr)
AA* 101.60 4.000 6225.52 219.6 96080
* 76.20 3.000 2626.39 92.64 40530
½* 53.45 2.103 907.18 32.000 14000
A* 50.80 2.000 778.19 27.45 12010
1* 42.42 1.669 453.59 16.000 7000
* 37.05 1.459 302.39 10.667 4667
2* 33.67 1.325 226.80 8.000 3500
3* 29.41 1.158 151.20 5.333 2333
4 26.72 1.052 113.40 4.000 1750
23.75 to 24.25 (Euro) .935 to .955 (Euro)
B* 25.40 1.000 97.27 3.43 1501
6 23.35 .919 75.60 2.667 1166
8 21.21 .835 56.70 2.000 875
10 19.69 .775 45.36 1.600 700
12 18.53 .729 37.80 1.333 583
13 18.04 .710 34.89 1.231 538
14 17.60 .693 32.40 1.143 500
16 16.83 .663 28.35 1.000 438
20 15.63 .615 22.68 0.800 350
24 14.70 .579 18.90 0.667 292
28 13.97 .550 16.20 0.571 250
32 13.36 .526 14.17 0.500 219
36 12.85 .506 12.59 0.444 194
C* 12.70 .500 12.16 0.429 188
40 12.40 .488 11.34 0.400 175
67½ 10.41 .410 6.71 0.237 104
D* 6.35 .250 1.52 0.0536 23.5

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) bullet, or sometimes slightly heavier, rather than using a 4 ounce round lead ball. (Round balls give progressively much worse external ballistic performance than conical bullets at ranges greater than about 75 yards.) In contrast, a 4-bore express rifle often used a 1,500-grain (3.43 oz) 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.


  1. ^ Clair Rees (March, 2000). "Marlin's "Garden Gun" - Model 25MG". Guns Magazine. 
  2. ^ Frank C. Barnes, ed. Stan Skinner (2003). Cartridges of the World, 10th Ed.. Krause Publications. ISBN 0-87349-605-1. 
  3. ^ Randy Wakeman. "Why The 10 Gauge Shotgun is Obsolete". 

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