Gauge (firearms)

Not to be confused with caliber.

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

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) of the barrel.

Gauge is 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 (454 grams). 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 lb (3.6 kg) ball.

Gauge is commonly used today in reference to shotguns, though historically it was also used in large double rifles, which were made in sizes up to 2 bore during their heyday in the 1880s, being originally loaded with black powder cartridges. These very large rifles, called "elephant guns", were intended for use primarily in Africa and Asia for hunting large dangerous game.

Gauge is commonly abbreviated as "ga.", "ga", or "G".

Calculating gauge

An n-gauge diameter means that a ball of lead (density 11.34 g/cm³ or 0.4097 lb/in³) with that diameter has a mass equal to ⁠1/n⁠ part of the mass of the international avoirdupois pound (approx. 454 grams), that is, that n such lead balls could be cast from a pound weight of lead. Therefore, an n-gauge shotgun or n-bore rifle has a bore diameter (in inches) of approximately

$$d_{n}=2{\sqrt[{3}]{{\frac {3}{4\pi }}{\frac {1\mathrm {~lb} /n}{0.4097\mathrm {~lb/in^{3}} }}}}$$

Explanation:

• Divide 1 pound by n to find the mass of each one of the balls
• Divide it by 0.4097 lb/in³ (density of lead) to find the volume of each ball
• Multiply it by ⁠3/4⁠ and divide it by πぱい, then find its cube root (rearranged from the volume of a sphere equation) to find the radius of each ball
• Multiply it by 2 to change from radius to diameter

This simplifies to the following formula for the internal diameter of the barrel of an n-gauge shotgun:

$$d_{n}=1.67/{\sqrt[{3}]{n}}$$

in inches, or

$$d_{n}=42.4/{\sqrt[{3}]{n}}$$

in millimeters.

Likewise, given the diameter in inches, the gauge is $$n=4.66/d_{n}^{3}$$

The gauge of firearms is determined by: 1 pound/gauge = weight of lead sphere. Caliber of the bore is then measured.

1 Pound / gauge = weight of lead sphere		Diameter of bore is then measured
gauge	pounds	mm	inches
0.25	4	67.34	2.651
0.5	2	53.45	2.103
0.75	⁠1+1/3⁠	46.70	1.838
1	1	42.42	1.669
1.5	⁠2/3⁠	37.05	1.459
2	⁠1/2⁠	33.67	1.326
3	⁠1/3⁠	29.41	1.158
4	⁠1/4⁠	26.72	1.052
5	⁠1/5⁠	24.80	0.976
6	⁠1/6⁠	23.35	0.919
6.278	⁠1/6.278⁠	23.00	0.906
7	⁠1/7⁠	22.18	0.873
8	⁠1/8⁠	21.21	0.835
9	⁠1/9⁠	20.39	0.803
10	⁠1/10⁠	19.69	0.775
11	⁠1/11⁠	19.07	0.751
12	⁠1/12⁠	18.53	0.729
13	⁠1/13⁠	18.04	0.710
14	⁠1/14⁠	17.60	0.693
15	⁠1/15⁠	17.21	0.677
16	⁠1/16⁠	16.83	0.663
17	⁠1/17⁠	16.50	0.650
18	⁠1/18⁠	16.19	0.637
19	⁠1/19⁠	15.90	0.626
20	⁠1/20⁠	15.63	0.615
21	⁠1/21⁠	15.37	0.605
22	⁠1/22⁠	15.13	0.596
23	⁠1/23⁠	14.91	0.587
24	⁠1/24⁠	14.70	0.579
25	⁠1/25⁠	14.50	0.571
26	⁠1/26⁠	14.31	0.564
27	⁠1/27⁠	14.12	0.556
28	⁠1/28⁠	13.97	0.550
29	⁠1/29⁠	13.79	0.543
30	⁠1/30⁠	13.64	0.537
31	⁠1/31⁠	13.49	0.531
32	⁠1/32⁠	13.36	0.526
33	⁠1/33⁠	13.20	0.520
34	⁠1/34⁠	13.08	0.515
35	⁠1/35⁠	12.95	0.510
36	⁠1/36⁠	12.85	0.506
37	⁠1/37⁠	12.73	0.501
38	⁠1/38⁠	12.62	0.497
39	⁠1/39⁠	12.50	0.492
40	⁠1/40⁠	12.40	0.488
41	⁠1/41⁠	12.30	0.484
42	⁠1/42⁠	12.19	0.480
43	⁠1/43⁠	12.16	0.477
44	⁠1/44⁠	12.01	0.473
45	⁠1/45⁠	11.91	0.469
46	⁠1/46⁠	11.84	0.466
47	⁠1/47⁠	11.80	0.463
48	⁠1/48⁠	11.66	0.459
49	⁠1/49⁠	11.58	0.456
50	⁠1/50⁠	11.51	0.453
51	⁠1/51⁠	11.43	0.450
52	⁠1/52⁠	11.35	0.447
53	⁠1/53⁠	11.30	0.445
54	⁠1/54⁠	11.23	0.442
55	⁠1/55⁠	11.15	0.439
56	⁠1/56⁠	11.07	0.436
57	⁠1/57⁠	11.02	0.434
58	⁠1/58⁠	10.95	0.431
59	⁠1/59⁠	10.90	0.429
60	⁠1/60⁠	10.85	0.427
61	⁠1/61⁠	10.77	0.424
62	⁠1/62⁠	10.72	0.422
63	⁠1/63⁠	10.67	0.420
64	⁠1/64⁠	10.59	0.417
65	⁠1/65⁠	10.54	0.415
66	⁠1/66⁠	10.49	0.413
67	⁠1/67⁠	10.44	0.411
67.62	⁠1/67.62⁠	10.41	0.410
68	⁠1/68⁠	10.40	0.409
69	⁠1/69⁠	10.34	0.407
70	⁠1/70⁠	10.29	0.405
71	⁠1/71⁠	10.24	0.403
72	⁠1/72⁠	10.20	0.401
73	⁠1/73⁠	10.16	0.400
74	⁠1/74⁠	10.10	0.3978
75	⁠1/75⁠	10.05	0.3960
76	⁠1/76⁠	10.01	0.3942
77	⁠1/77⁠	9.97	0.3925
78	⁠1/78⁠	9.93	0.3908
79	⁠1/79⁠	9.86	0.3892
80	⁠1/80⁠	9.84	0.3876
81	⁠1/81⁠	9.80	0.3860
82	⁠1/82⁠	9.76	0.3844
83	⁠1/83⁠	9.72	0.3828
84	⁠1/84⁠	9.67	0.3813
85	⁠1/85⁠	9.65	0.3798
86	⁠1/86⁠	9.61	0.3783
87	⁠1/87⁠	9.57	0.3769
88	⁠1/88⁠	9.54	0.3754
89	⁠1/89⁠	9.50	0.3740
90	⁠1/90⁠	9.46	0.3726
91	⁠1/91⁠	9.43	0.3713
92	⁠1/92⁠	9.40	0.3699
93	⁠1/93⁠	9.36	0.3686
94	⁠1/94⁠	9.33	0.3673
95	⁠1/95⁠	9.30	0.3660
96	⁠1/96⁠	9.26	0.3647
97	⁠1/97⁠	9.23	0.3634
98	⁠1/98⁠	9.20	0.3622
99	⁠1/99⁠	9.17	0.3610
100	⁠1/100⁠	9.14	0.3598

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 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 (0.73 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

See also: Shotgun § Gauge

left-to-right: .410 bore, 28 gauge, 20 gauge, and 12 gauge shotgun shells

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, being favored by shooters uncomfortable with the weight and recoil 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.

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.

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.

Gauge and shot type

A 10-gauge (3+1⁄2-inch (89 mm)) shotgun shell shown next to a United States quarter

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

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
	(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.