Gauge (firearms)

Gauge
Gauge
	From left to right; a .45 ACP cartridge, a .410 bore shotshell, a 20-gauge shotshell, and a 12-gauge shotshell
Type	Shotgun
Place of origin	Various

Last updated January 14, 2025

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)^[1] of The London Proof House and The Birmingham Proof House, as referred in the Gun Barrel Proof Act 1978, Paragraph 6;^[2] 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.^[3]

The term gauge in connection of firearms was first used in the book, A Light to the Art of Gunnery (1677).^[4]

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).^[5] 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.^[6]

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.^[7] By far the most popular is the 12 gauge,^[7] particularly in the United States.^[8] The 20-gauge shotgun is the next most popular size, being favored by shooters uncomfortable with the weight of a 12-gauge gun, 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.^{[ citation needed ]}^[9]

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;^[10] 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.^[10] 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.

Garden gun calibers: 9mm Flobert shot, 9mm Flobert shot, .22 Long Rifle shot, .22 Long Rifle, .22 Long Rifle shot, .22 CB Short, and 9mm Flobert BB cap Munit05.jpg — Garden gun calibers: 9mm Flobert shot, 9mm Flobert shot, .22 Long Rifle shot, .22 Long Rifle, .22 Long Rifle shot, .22 CB Short, and 9mm Flobert BB cap

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.^[11]^[12]

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).^[13] 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.^[14] However, laboratory research indicates that tungsten alloys can actually be quite toxic internally.^[15]

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

Portrait of Frederick Courteney Selous with his 4 bore single-shot Boer rifle and African hunting regalia, 1876 YoungFCSelous.jpg — Portrait of Frederick Courteney Selous with his 4 bore single-shot Boer rifle and African hunting regalia, 1876

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		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.

Related Research Articles

Handloading, or reloading, is the practice of making firearm cartridges by manually assembling the individual components, rather than purchasing mass-assembled, factory-loaded commercial ammunition.

A shotgun is a long-barreled firearm designed to shoot a straight-walled cartridge known as a shotshell, which discharges numerous small spherical projectiles called shot, or a single solid projectile called a slug. Shotguns are most commonly used as smoothbore firearms, meaning that their gun barrels have no rifling on the inner wall, but rifled barrels for shooting sabot slugs are also available.

A cartridge, also known as a round, is a type of pre-assembled firearm ammunition packaging a projectile, a propellant substance and an ignition device (primer) within a metallic, paper, or plastic case that is precisely made to fit within the barrel chamber of a breechloading gun, for convenient transportation and handling during shooting. Although in popular usage the term "bullet" is often used to refer to a complete cartridge, the correct usage only refers to the projectile.

In guns, particularly firearms, but not artillery, where a different definition may apply, caliber is the specified nominal internal diameter of the gun barrel bore – regardless of how or where the bore is measured and whether the finished bore matches that specification. It is measured in inches or in millimeters. In the United States it is expressed in hundredths of an inch; in the United Kingdom in thousandths; and elsewhere in millimeters. For example, a US "45 caliber" firearm has a barrel diameter of roughly 0.45 inches (11.43mm). Barrel diameters can also be expressed using metric dimensions. For example, a "9 mm pistol" has a barrel diameter of about 9 millimeters. Since metric and US customary units do not convert evenly at this scale, metric conversions of caliber measured in decimal inches are typically approximations of the precise specifications in non-metric units, and vice versa.

A rim-fire is a type of metallic cartridge used in firearms where the primer is located within a hollow circumferential rim protruding from the base of its casing. When fired, the gun's firing pin will strike and crush the rim against the edge of the barrel breech, sparking the primer compound within the rim, and in turn ignite the propellant within the case. Invented in 1845 by Louis-Nicolas Flobert, the first rimfire metallic cartridge was the .22 BB Cap cartridge, which consisted of a percussion cap with a bullet attached to the top. While many other different cartridge priming methods have been tried since the early 19th century, such as teat-fire and pinfire, only small caliber rimfire cartridges have survived to the present day with regular use. The .22 Long Rifle rimfire cartridge, introduced in 1887, is by far the most common ammunition found in the world today in terms of units manufactured and sold.

A slug barrel is a shotgun barrel that is designed primarily to fire slugs instead of shot, and is usually rifled. A shotgun with such a barrel is often called a slug gun.

A shotgun cartridge, shotshell, or shell is a type of rimmed, cylindrical (straight-walled) ammunition used specifically in shotguns. It is typically loaded with numerous small, spherical sub-projectiles called shot. Shotguns typically use a smoothbore barrel with a tapered constriction at the muzzle to regulate the extent of scattering.

The .410 bore (10.4 mm) is one of the smallest caliber of shotgun shell commonly available. A .410 bore shotgun loaded with shot shells is well suited for small game hunting and pest control. The .410 started off in the United Kingdom as a garden gun along with the .360 and the No. 3 bore (9 mm) rimfire, No. 2 bore (7 mm) rimfire, and No. 1 bore (6 mm) rimfire. .410 shells have similar base dimensions to the .45 Colt cartridge, allowing many single-shot firearms, as well as derringers and revolvers chambered in that caliber, to fire .410 shot shells without any modifications.

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Garden guns are small bore shotguns commonly used by gardeners and farmers for pest control. They are made to fire small gauges such as .410 bore, .360 bore, 9mm Flobert, and .22 Winchester Magnum Rimfire, or .22 Long Rifle rimfire shotshell cartridges. They are short-range shotguns that can do little harm past 15 to 20 yd, and they are relatively quiet when fired with rimfire ammunition. These guns are especially suitable for use inside barns and sheds, as the low-velocity small shot will not penetrate roofs or walls, nor injure livestock with a ricochet. Such guns are also used for pest control at airports, warehouses, stockyards, etc.

References

↑ "RULES, REGULATIONS AND SCALES". 2006.{{cite web}}: CS1 maint: url-status (link)
↑ "Gun Barrel Proof Act 1978". 1978.
↑ van Meteren, Emanuel (1599). Belgische ofte Nederlantsche historie, van onsen tijden [A Generall Historie of the Netherlands: with the genealogie and memorable acts of the Earls of Holland, Zeeland, and West-Friseland, from Thierry of Aquitaine the first Earle, successively unto Philip the Third, King of Spain] (in Dutch). Koninklijke Bibliotheek, Nationale bibliotheek van Nederland: Vennecool, Jacob Conelissz. p. 843.
↑ Binngin, Thomas (1677). A Light to the Art of Gunnery. p. 175.
↑ Barnes, Frank C.; Woodard, W. Todd (2016). Cartridges of the world : a complete and illustrated reference for more than 1500 cartridges (15th ed.). Krause Publications. p. 629. ISBN 978-1440246425. OCLC 934886116.
↑ Gun Barrel Proof Act 1855
1 2 Krause, Angel. "What Shotgun Gauge Should I Use for Hunting?". Outly. Retrieved 27 November 2021.
↑ Carter, Greg Lee (2002). Guns in American Society: An Encyclopedia. Santa Barbara, Calif.; Oxford: ABC-CLIO. p. 361. ISBN 978-1-57607-268-4.
↑ Wintersteen, K. (2014, December 9). The decline of 10- & 16-gauge shotguns. Firearms News. https://www.firearmsnews.com/editorial/decline-10-16-gauge-shotguns/78028
1 2 Frank C. Barnes (2009). Layne Simpson (ed.). Cartridges of the World (12th ed.).
↑ Clair Rees (March 2000). "Marlin's 'Garden Gun'—Model 25MG". Guns Magazine. Archived from the original on 2008-04-29. Retrieved 2008-05-08.
↑ Frank C. Barnes (2003). Stan Skinner (ed.). Cartridges of the World (10th ed.). Krause Publications. ISBN 0-87349-605-1.
↑ Tom Roster is an independent ballistics consultant and author specializing in the design and testing of shotshell loads for U.S. shotshell and reloading components manufacturers. He is a court-recognized shotshell/shotgun expert witness. Tom was formerly, T. R. (2014, January 1). Important information about shotshell pressures. Shotgun Life. https://www.shotgunlife.com/shotguns/tom-roster/important-information-about-shotshell-pressures.html
↑ Randy Wakeman (2007). "Why the 10 Gauge Shotgun Is Obsolete". chuckhawks.com. Archived from the original on 2006-05-13.
↑ John Kalinich; et al. (2005). "Embedded weapons-grade tungsten alloy shrapnel rapidly induces metastatic high-grade rhabdomyosarcomas in F344 rats". Environmental Health Perspectives. 113 (6): 729–34. Bibcode:2005EnvHP.113..729K. doi:10.1289/ehp.7791. PMC 1257598 . PMID 15929896.

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[:0-1] "RULES, REGULATIONS AND SCALES". 2006.{{cite web}}: CS1 maint: url-status (link)

[2] "Gun Barrel Proof Act 1978". 1978.

[3] van Meteren, Emanuel (1599). Belgische ofte Nederlantsche historie, van onsen tijden [A Generall Historie of the Netherlands: with the genealogie and memorable acts of the Earls of Holland, Zeeland, and West-Friseland, from Thierry of Aquitaine the first Earle, successively unto Philip the Third, King of Spain] (in Dutch). Koninklijke Bibliotheek, Nationale bibliotheek van Nederland: Vennecool, Jacob Conelissz. p. 843.

[4] Binngin, Thomas (1677). A Light to the Art of Gunnery. p. 175.

[5] Barnes, Frank C.; Woodard, W. Todd (2016). Cartridges of the world : a complete and illustrated reference for more than 1500 cartridges (15th ed.). Krause Publications. p. 629. ISBN 978-1440246425. OCLC 934886116.

[6] Gun Barrel Proof Act 1855

[Outly-7] 1 2 Krause, Angel. "What Shotgun Gauge Should I Use for Hunting?". Outly. Retrieved 27 November 2021.

[8] Carter, Greg Lee (2002). Guns in American Society: An Encyclopedia. Santa Barbara, Calif.; Oxford: ABC-CLIO. p. 361. ISBN 978-1-57607-268-4.

[9] Wintersteen, K. (2014, December 9). The decline of 10- & 16-gauge shotguns. Firearms News. https://www.firearmsnews.com/editorial/decline-10-16-gauge-shotguns/78028

[Frank_C._Barnes_2009-10] 1 2 Frank C. Barnes (2009). Layne Simpson (ed.). Cartridges of the World (12th ed.).

[11] Clair Rees (March 2000). "Marlin's 'Garden Gun'—Model 25MG". Guns Magazine. Archived from the original on 2008-04-29. Retrieved 2008-05-08.

[cotw10-12] Frank C. Barnes (2003). Stan Skinner (ed.). Cartridges of the World (10th ed.). Krause Publications. ISBN 0-87349-605-1.

[13] Tom Roster is an independent ballistics consultant and author specializing in the design and testing of shotshell loads for U.S. shotshell and reloading components manufacturers. He is a court-recognized shotshell/shotgun expert witness. Tom was formerly, T. R. (2014, January 1). Important information about shotshell pressures. Shotgun Life. https://www.shotgunlife.com/shotguns/tom-roster/important-information-about-shotshell-pressures.html

[14] Randy Wakeman (2007). "Why the 10 Gauge Shotgun Is Obsolete". chuckhawks.com. Archived from the original on 2006-05-13.

[15] John Kalinich; et al. (2005). "Embedded weapons-grade tungsten alloy shrapnel rapidly induces metastatic high-grade rhabdomyosarcomas in F344 rats". Environmental Health Perspectives. 113 (6): 729–34. Bibcode:2005EnvHP.113..729K. doi:10.1289/ehp.7791. PMC 1257598 . PMID 15929896.

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