Gauge (firearms)

with his 4 bore single-shot Boer rifle and African hunting regalia, 1876 The concept of using a material property to define a bore diameter was used before the term "gauge" in the late 16th century. The term gauge in connection of firearms was first used in the book A Light to the Art of Gunnery (1677). Gauge was determined from the weight of a solid sphere of lead that would fit the bore of the firearm and is expressed as the multiplicative inverse of the sphere's mass as a fraction of a pound, e.g., a lead ball fits a 12-gauge bore. Therefore, with a 12-gauge, it would take 12 balls of lead of the same size as the 12-gauge shotgun's inner bore diameter to mass . The term is related to the measurement of cannons, which were also measured by the weight of their iron round shot; an eight-pounder would fire an ball. Therefore, a 12-gauge is larger than a 16-gauge. Since the term was defined in terms of pounds and inches, this implied that for a density of lead in , the formula for the bore diameter in inches based on the gauge was: \begin{align} b &= 2\sqrt[3]{\frac{3}{4\pi g\rho}} &&\approx 2\sqrt[3]{\frac{3}{1.64\pi g}} &&&\approx 1.67\sqrt[3]{\frac{1}{g}}\\ \rho &\approx 0.41 \frac{\mathrm{lb}_{\mathrm{m}}}{\mathrm{in}^3} \end{align} Due to problems defining a pound, and to get pure lead, the Gun Barrel Proof Act of 1855 defined a gauge as a list of defined values. As a result, the formula above only works as a heuristic for determining bore diameter from gauge, as the table in the law lists diameters not consistent with any possible density, even after accounting for rounding errors inherent in the table only being precise to a hundredth of an inch. Gauge is commonly used today in reference to shotguns, though historically it was first used in muzzle-loading long guns such as muskets, then later on in breech-loading long guns including single-shot and double rifles, which were made in sizes up to 2 bore during their heyday in the mid to late 19th century, being originally loaded as black powder cartridges. These very large and heavy rifles, called "elephant guns", were intended primarily for use in regions of Africa and Asia to hunt large, dangerous game animals. Gauge is commonly abbreviated as "ga.", "ga", or "G". Legal Definition The British law that defined the relationship between bore diameter and gauge listed every gauge from 1 to 50. The list here does the same: the number in the list is the gauge. The value to its right is the diameter in inches, as specified in the law. For convenience, the equivalent value in millimetres is provided, but this is not specified in the law. ==Bore sizing==

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 a few inches on some firearms. 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 , can range from a tight to an extreme overbore of . Some also claim an increased velocity with the overbored barrels, up to , 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 performed to reduce barrel weight and shift the center of mass backward for better balance. Factory overbored barrels generally have a larger outside diameter and will not have this weight reduction, though they are tougher because 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. ==Gauges in use==

Shotgun gauges still in modern production include, in descending order of size, 10 gauge, 12 gauge, 16 gauge, 20 gauge, 24 gauge, 28 gauge, 32 gauge (sometimes called 14 millimeter or 32 bore), and .410 bore. By far, the most popular is the 12 gauge, Shotguns and shells exceeding 10 gauge, such as the 8 gauge, 6 gauge, 4 gauge, and 2 gauge, are historically important in the United Kingdom and elsewhere in mainland Europe. Today, they are almost exclusively made by custom order or handloaders. These shells are usually black powder paper or brass cartridges, as opposed to modern smokeless powder plastic or wax cartridges. The 18-, 15-, 11-, 6-, 3-, and 2-gauge shells are the rarest of all; owners of these types of rare shotguns will usually have their ammunition custom-loaded by a specialist in rare and custom bores. The 14 gauge has not been loaded in the United States since the early 20th century, although the hull is still made in France. Gauge and shot type The 10 gauge narrowly escaped obsolescence when steel and other nontoxic shot became required for waterfowl hunting, since the larger shell could hold the much larger sizes of low-density steel shot needed to reach the ranges necessary for waterfowl hunting. The move to steel shot reduced the use of 16- and 20-gauge shells for waterfowl hunting, and the shorter, , 12-gauge shells as well. However, the 12-gauge shell, with its higher SAAMI pressure rating of compared to standard and 12-gauge shells with their lower pressure rating of , began to approach the performance of the 10-gauge shells with a pressure rating of . Newer nontoxic shots, such as bismuth or tungsten-nickel-iron alloys, and even tungsten-polymer blends, regain much or all of the performance loss, but are much more expensive than steel or lead shot. ==References==