# Gauge (firearms)

The gauge (in American English or more commonly referred to as bore in British English) of a firearm is a unit of measurement used to express the inner diameter (bore diameter) and other necessary parameters to define in general a smoothbore barrel (compare to caliber, which defines a barrel with rifling and its cartridge).
The gauge of a shotgun is a list that includes all necessary data to define a functional barrel. For example, the dimension of the chamber, the shotgun bore dimension and the valid proof load and commercial ammunition, as defined globally by the C.I.P.; defined in Great Britain by the Rules, regulations and scales applicable to the proof of small arms (2006) of The London Proof House and The Birmingham Proof House, as referred in the Gun Barrel Proof Act 1978, Paragraph 6; and defined in the United States by SAAMI Z299.2 – 2019.

## Historical development

The concept of using a material property to define a bore diameter was used before the term gauge, in the end of the 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 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 one-twelfth pound 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 weigh 1 pound (453.6 g). 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 8-pound (3.6 kg) ball. Therefore, a 12 gauge is larger than a 16 gauge.
Due to problems defining a pound, and to get pure lead, the Gun Barrel Proof Act 1855 defined a gauge as a list of defined values.
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 for use primarily in regions of Africa and Asia for hunting large dangerous game animals.
Gauge is commonly abbreviated as "ga.", "ga", or "G".

## 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 18.5 mm (0.73 in), can range from a tight 18 mm (0.71 in) to an extreme overbore of 20 mm (0.79 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 backward 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.

## Gauges in use

The six most common shotgun gauges, in descending order of size, are the 10 gauge, 12 gauge, 16 gauge, 20 gauge, 28 gauge, and .410 bore. By far the most popular is the 12 gauge, particularly in the United States. The 20-gauge shotgun is the next most popular size, and is popular for upland game hunting. The next most popular sizes are the .410 bore and the 28 gauge. The least popular sizes are the 10 gauge and the 16 gauge; while far less common than the other four gauges, they are still commercially available.
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 rarely manufactured. 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 2+9⁄16-inch (65 mm) hull is still made in France. The very small 24 and 32 gauges are still produced and used in some European and South American countries. Punt guns, which use very large shells, are rarely encountered.
Also seen in limited numbers are smoothbore firearms in calibers smaller than .360 such as .22 Long Rifle (UK No. 1 bore) and 9mm Flobert rimfire (UK No. 3 bore), designed for short-range pest control and garden guns. The No. 2 bore (7 mm) has long been obsolete. All three of these rimfires are available in shot and BB-cap.

### 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 gauges for waterfowl hunting, and the shorter, 2+3⁄4-inch (70 mm), 12-gauge shells as well. However, the 3+1⁄2-inch (89 mm) 12-gauge shell, with its higher SAAMI pressure rating of 14,000 psi (97 MPa) compared to standard 2+3⁄4-inch (70 mm) and 3-inch (76 mm) 12-gauge shells with their lower pressure rating of 11,500 psi (79 MPa), began to approach the performance of the 3+1⁄2-inch (89 mm) 10-gauge shells with a pressure rating of 11,000 psi (76 MPa). 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. However, laboratory research indicates that tungsten alloys can actually be quite toxic internally.

### Bore sizes used in the United Kingdom

Legend: left side is the bore size, right side is the case length
* 2 bore: 3+1⁄2 in (89 mm)
* 4 bore: 3+1⁄4, 4 in (83, 102 mm)
* 6 bore
* 8 bore: 3, 3+1⁄4, 3+3⁄4, 4 in (76, 83, 95, 102 mm)
* 10 bore: 2+9⁄16, 2+5⁄8, 2+7⁄8, 3, 3+1⁄4, 3+1⁄2 in (65, 67, 73, 76, 83, 89 mm)
* 12 bore: 1+3⁄4, 2, 2+1⁄4, 2+1⁄2, 2+3⁄4, 3, 3+1⁄2 in (44, 51, 57, 64, 70, 76, 89 mm)
* 14 bore: 2+1⁄2 in (64 mm)
* 16 bore: 2+1⁄2, 2+9⁄16, 2+5⁄8, 2+3⁄4, 3 in (64, 65, 67, 70, 76 mm)
* 20 bore: 2+1⁄2, 2+3⁄4, 3, 3+1⁄2 in (64, 70, 76, 89 mm)
* 24 bore: 2+1⁄2 in (64 mm)
* 28 bore: 2+1⁄2, 2+3⁄4, 3 in (64, 70, 76 mm)
* 32 bore: 2+1⁄2 in (64 mm)
* .410 bore: 2, 2+1⁄2, 3 in (51, 64, 76 mm)
* .360 bore: 1+3⁄4 in (44 mm)
* 9 mm (No. 3 bore) short rimfire, 9 mm (No. 3 bore) long rimfire
* 7 mm (No. 2 bore) rimfire
* 6 mm (No. 1 bore) short rimfire, 6 mm (No. 1 bore) long rimfire

## Conversion guide

The table below lists various gauge sizes with weights. The bores marked $^*$ are found in punt guns, obsolete, or rare weapons only. However, 4 gauge was sometimes found used in blunderbuss guns made for coach defense and protection against piracy. The .410 and 23 mm are exceptions; they are actual bore sizes, not gauges. If the .410 bore and 23 mm diameters were measured using more traditional means, they would be equivalent to 67.62 gauge (.410 bore) and 6.278 gauge (23 mm), respectively.
| Gauge (bore) | Diameter | Diameter | Weight of unalloyed (pure) lead ball | Weight of unalloyed (pure) lead ball | Weight of unalloyed (pure) lead ball |
| Gauge (bore) | (mm) | (in) | grams | ounces | grains |
| AA$^*$ | 101.60 | 4.000 | 6,225.52 | 219.6 | 96,080 |
| A⁠1/2⁠$^*$ | 76.20 | 3.000 | 2,626.39 | 92.64 | 40,530 |
| 0.25$^*$ | 67.34 | 2.651 | 1,814.36 | 64.000 | 28,000 |
| 0.5$^*$ | 53.45 | 2.103 | 907.18 | 32.000 | 14,000 |
| A$^*$ | 50.80 | 2.000 | 778.19 | 27.45 | 12,010 |
| 0.75$^*$ | 46.70 | 1.838 | 604.80 | 21.336 | 9,328 |
| 1$^*$ | 42.42 | 1.669 | 453.59 | 16.000 | 7,000 |
| B⁠1/2⁠$^*$ | 38.10 | 1.500 | 328.3 | 11.58 | 5,066 |
| 1.5$^*$ | 37.05 | 1.459 | 302.39 | 10.667 | 4,667 |
| 2$^*$ | 33.67 | 1.326 | 226.80 | 8.000 | 3,500 |
| 3$^*$ | 29.41 | 1.158 | 151.20 | 5.333 | 2,333 |
| 4$^*$ | 26.72 | 1.052 | 113.40 | 4.000 | 1,750 |
| B$^*$ | 25.40 | 1.000 | 97.27 | 3.43 | 1,501 |
| 5$^*$ | 24.80 | 0.976 | 90.72 | 3.200 | 1,400 |
| 6$^*$ | 23.35 | 0.919 | 75.60 | 2.667 | 1,166 |
| 6.278$^*$ | 23.00 | 0.906 | 72.26 | 2.549 | 1,114 |
| 7$^*$ | 22.18 | 0.873 | 64.80 | 2.286 | 1,000 |
| 8$^*$ | 21.21 | 0.835 | 56.70 | 2.000 | 875 |
| 9$^*$ | 20.39 | 0.803 | 50.40 | 1.778 | 778 |
| 10 | 19.69 | 0.775 | 45.36 | 1.600 | 700 |
| 11$^*$ | 19.07 | 0.751 | 41.24 | 1.454 | 636 |
| 12 | 18.53 | 0.729 | 37.80 | 1.333 | 583 |
| 13$^*$ | 18.04 | 0.710 | 34.89 | 1.231 | 538 |
| 14$^*$ | 17.60 | 0.693 | 32.40 | 1.143 | 500 |
| 15$^*$ | 17.21 | 0.677 | 30.24 | 1.067 | 467 |
| 16 | 16.83 | 0.663 | 28.35 | 1.000 | 438 |
| 17$^*$ | 16.50 | 0.650 | 26.68 | 0.941 | 412 |
| 18$^*$ | 16.19 | 0.637 | 25.20 | 0.889 | 389 |
| 20 | 15.63 | 0.615 | 22.68 | 0.800 | 350 |
| 22$^*$ | 15.13 | 0.596 | 20.62 | 0.728 | 319 |
| 24 | 14.70 | 0.579 | 18.90 | 0.667 | 292 |
| 26$^*$ | 14.31 | 0.564 | 17.44 | 0.615 | 269 |
| 28 | 13.97 | 0.550 | 16.20 | 0.571 | 250 |
| 32 | 13.36 | 0.526 | 14.17 | 0.500 | 219 |
| 36$^*$ | 12.85 | 0.506 | 12.59 | 0.444 | 194 |
| 40$^*$ | 12.40 | 0.488 | 11.34 | 0.400 | 175 |
| 67.62 | 10.41 | 0.410 | 6.71 | 0.237 | 104 |
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; 129.60 g) bullet, or sometimes slightly heavier, rather than using a 4-ounce (110 g) round lead ball. (Round balls lose velocity faster than conical bullets and have much steeper ballistic trajectories beyond about 75 yards or 69 metres) In contrast, a 4-bore express rifle often used a 1,500-grain (3.43 oz; 97.20 g) bullet wrapped in paper to keep lead buildup to a minimum in the barrel. In either case, assuming a 4-ounce (110 g) mass for a 4-bore rifle bullet from this table would be inaccurate, although indicative.