Gauge (bore diameter)

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This article is about the gauge measurement of firearms. For other uses see gauge.

The Gauge or bore (especially in British English) of a shotgun is the diameter (caliber) of the barrel.

Contents

[edit] Calculating gauge

The gauge or bore is determined by the number of solid spheres of a diameter equal to the inside diameter of the barrel that could be made from a pound of lead. The term related to the measurement of black powder cannon, which were also measured by the weight of their round iron shot; a 6 pounder, for example, would fire a 6 pound (2.7 kg) spherical cast iron ball, which gave a bore diameter of about 3.6 inches (9.1 cm).

An n-gauge diameter means that n balls of lead (density 11.352 g/cm³) with that diameter weigh one pound (453.5924 g). Therefore an n-gauge shotgun has a bore diameter 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} = 42.416~\mathrm{mm} \times n^{-1/3}

[edit] Gauge in use

From left to right: 9 mm Luger Parabellum, .40 S&W, .45 ACP, 5.7x28mm, 5.56x45mm NATO, .300 Winchester Magnum, and a 2.75-inch and 3-inch 12 gauge.
From left to right: 9 mm Luger Parabellum, .40 S&W, .45 ACP, 5.7x28mm, 5.56x45mm 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 4 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 0.729 inches, can range from a tight 0.720 inches to an extreme overbore of 0.800 inches. Some also claim an increased velocity with the overbored barrels, up to 50 feet per second, which is due to the larger swept area 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.

Note: Slugs should not be fired from overbored barrels, as the slug may be incapable of obturating to fill the oversized bore, which can result in very inconsistent accuracy.

[edit] Sizes in use

Certain sizes are more common than others; 12-gauge is probably the most common size. The 20-gauge is popular with shooters who are uncomfortable with the weight and recoil of a 12 gauge gun. The next most popular size is the .410, which is not a gauge, but a caliber. 10, 16 and 28 gauges, while less common, are still readily available.

[edit] Conversion guide

A table showing the various shotgun 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.

Gauge
(Bore)		 Caliber Weight of lead ball
(in) (mm) (oz) (gr) (g)
1½* 1.459 37.05 10.667 4667 302.39
2* 1.325 33.67 8.000 3500 226.80
3* 1.158 29.41 5.333 4667 151.20
4 1.052 26.72 4.000 1750 113.40
.935 to .955 23.75 to 24.25
8 .835 21.21 2.000 875 56.70
10 .775 19.69 1.600 700 45.36
12 .729 18.53 1.333 583 37.80
13 .710 18.04 1.231 538 34.89
14 .693 17.60 1.143 500 32.40
16 .663 16.83 1.000 438 28.35
20 .615 16.53 0.800 350 22.68
24 .579 14.70 0.667 292 18.90
28 .550 13.97 0.571 250 16.20
32 .526 13.36 0.500 219 14.17
67½ .410 10.41 0.237 104 6.71

[edit] See also

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