Gauge (bore diameter)

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

"12 gauge" redirects here. For the rapper, see 12 Gauge (rapper).

From left to right; a .45 ACP, a .410 bore Shotshell, a 20 gauge shotshell, and a 12 gauge shotshell.

The Gauge or bore of a shotgun is a unit of measurement used to express the diameter of the barrel. The gauge or bore of the inside diameter of a barrel corresponds with the number of identical solid spheres that can be made from a pound of lead. The term also 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).

Calculating gauge

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 (in millimeters) of approximately

<math>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 \frac{1}{\sqrt[3]{n}}</math>

Gauge in use

From left to right: 9 mm Luger 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 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 18.5 mm (0.729 inches), can range from a tight 18.3 mm to an extreme overbore of 20.3 mm. 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 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. 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 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. Shotguns larger than 10-gauge are rarely manufactered nowadays; guns like the KS-23, a 4-gauge, are very few in number.

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.

See also

• Smoothbore
• Gauge
• Caliber
• .410 bore

References

External links

• Pacific Tool and Gauge