How The Shotgun Gauge System Works

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). 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) spherical cast iron ball and had a bore diameter of about 91 mm (3.6 in).
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

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

Gauges in use

From left to right: 9×19mm Parabellum, .40 S&W, .45 ACP, 5.7×28mm, 5.56×45mm 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 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 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 (0.73 in), can range from a tight 18.3 mm (0.72 in) to an extreme overbore of 20.3 mm (0.80 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 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

left-to-right:.410, 28ga, 20ga, 12ga

Certain sizes are more common than others; 12 gauge is the most common size,^[1] 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 the 10 gauge are rarely manufactured nowadays. 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. The very small 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.^[2][3]
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. The move to steel shot effectively shut out the 20 and 16 gauges, and smaller loads of 12 gauge as well. However, the advent of the 3.5 in (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.^[4]

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 67.5 gauge. The European 4-gauge (24 mm, nominal, caliber) is sized slightly different from the traditional 4-bore or 4-gauge, being based on the newer metric CIP tables.

Gauge (Bore)	Caliber		Weight of unalloyed (pure) lead ball
	(mm)	(in)	(g)	(oz)	(gr)
AA*	101.60	4.000	6225.52	219.6	96080
A½*	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
1½*	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.

References

1. ^ Carter, Greg Lee (2002). Guns In American Society: An Encyclopedia. ABC-CLIO. p. 361. ISBN 9781576072684. 
2. ^ Clair Rees (March, 2000). "Marlin's "Garden Gun"—Model 25MG". Guns Magazine. http://findarticles.com/p/articles/mi_m0BQY/is_3_46/ai_59281208. 
3. ^ Frank C. Barnes, ed. Stan Skinner (2003). Cartridges of the World, 10th Ed.. Krause Publications. ISBN 0-87349-605-1. 
4. ^ Randy Wakeman. "Why The 10 Gauge Shotgun is Obsolete". http://www.chuckhawks.com/10_gauge_obsolete.htm.