The Gauge or bore of a shotgun or rifle is a unit of measurement used to express the diameter of the barrel. The gauge or bore of a barrel is equal to the number of solid spheres of lead each having the same diameter as the inside of the barrel that would in total weigh a pound.
The term is 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 91 mm (3.6 inches).
Historically, large double rifles 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.
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 or n-bore rifle has a bore diameter (millimeters) of approximately
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 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.
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|
|23.75 to 24.25 (Euro)||.935 to .955 (Euro)|
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 2000 grain (4.57 ounce) 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 1500 grain (3.43 ounce) 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.