Gauge (firearms)

Last updated
Gauge
Shotshell 001.jpg
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 originVarious

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.

Contents

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). [1] 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

Explanation:

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

(in inches), or (in millimeters).

Likewise, given the diameter in inches, the gauge is

The gauge of firearms is determined by: 1 pound/gauge = weight of lead sphere. Caliber of the bore is then measured. [2]

1 Pound / gauge = weight of lead sphereDiameter of bore is then measured
gaugepoundsmminches
0.25467.342.651
0.5253.452.103
0.751+1/346.701.838
1142.421.669
1.52/337.051.459
21/233.671.326
31/329.411.158
41/426.721.052
51/524.800.976
61/623.350.919
6.2781/6.27823.000.906
71/722.180.873
81/821.210.835
91/920.390.803
101/1019.690.775
111/1119.070.751
121/1218.530.729
131/1318.040.710
141/1417.600.693
151/1517.210.677
161/1616.830.663
171/1716.500.650
181/1816.190.637
191/1915.900.626
201/2015.630.615
211/2115.370.605
221/2215.130.596
231/2314.910.587
241/2414.700.579
251/2514.500.571
261/2614.310.564
271/2714.120.556
281/2813.970.550
291/2913.790.543
301/3013.640.537
311/3113.490.531
321/3213.360.526
331/3313.200.520
341/3413.080.515
351/3512.950.510
361/3612.850.506
371/3712.730.501
381/3812.620.497
391/3912.500.492
401/4012.400.488
411/4112.300.484
421/4212.190.480
431/4312.160.477
441/4412.010.473
451/4511.910.469
461/4611.840.466
471/4711.800.463
481/4811.660.459
491/4911.580.456
501/5011.510.453
511/5111.430.450
521/5211.350.447
531/5311.300.445
541/5411.230.442
551/5511.150.439
561/5611.070.436
571/5711.020.434
581/5810.950.431
591/5910.900.429
601/6010.850.427
611/6110.770.424
621/6210.720.422
631/6310.670.420
641/6410.590.417
651/6510.540.415
661/6610.490.413
671/6710.440.411
67.621/67.6210.410.410
681/6810.400.409
691/6910.340.407
701/7010.290.405
711/7110.240.403
721/7210.200.401
731/7310.160.400
741/7410.100.3978
751/7510.050.3960
761/7610.010.3942
771/779.970.3925
781/789.930.3908
791/799.860.3892
801/809.840.3876
811/819.800.3860
821/829.760.3844
831/839.720.3828
841/849.670.3813
851/859.650.3798
861/869.610.3783
871/879.570.3769
881/889.540.3754
891/899.500.3740
901/909.460.3726
911/919.430.3713
921/929.400.3699
931/939.360.3686
941/949.330.3673
951/959.300.3660
961/969.260.3647
971/979.230.3634
981/989.200.3622
991/999.170.3610
1001/1009.140.3598

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

left-to-right: .410 bore, 28 gauge, 20 gauge, and 12 gauge shotgun shells Shotgun shell comparison.jpg
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. [3] By far the most popular is the 12 gauge, [3] particularly in the United States. [4] 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.[ citation needed ]

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. 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; [5] 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+916-inch (65 mm) hull is still made in France. [5] The very small 24 and 32 gauges are still produced and used in some European and South American countries. Punt guns 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. [6] [7]

Gauge and shot type

A 10-gauge (
3+1/2-inch (89 mm)) shotgun shell shown next to a United States quarter 10 Gauge Shell.jpg
A 10-gauge (3+12-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+34-inch (70 mm), 12-gauge shells as well. However, the 3+12-inch (89 mm) 12-gauge shell, with its higher SAAMI pressure rating of 14,000 psi (97 MPa) compared to standard 2+34-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+12-inch (89 mm) 10-gauge shells with a pressure rating of 11,000 psi (76 MPa). [8] 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. [9] However, laboratory research indicates that tungsten alloys can actually be quite toxic internally. [10]

Bore sizes used in the United Kingdom

Legend: left side is the bore size, right side is the case length

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 traditionally means, they would be equivalent to 67.62 gauge (.410 bore) and 6.278 gauge (23 mm), respectively.

Gauge
(bore)
DiameterWeight of unalloyed (pure) lead ball
(mm)(in)gramsouncesgrains
AA*101.604.0006,225.52219.696,080
*76.203.0002,626.3992.6440,530
0.25*67.342.6511,814.3664.00028,000
0.5*53.452.103907.1832.00014,000
A*50.802.000778.1927.4512,010
0.75*46.701.838604.8021.3369,328
1*42.421.669453.5916.0007,000
*38.101.500328.311.585,066
1.5*37.051.459302.3910.6674,667
2*33.671.326226.808.0003,500
3*29.411.158151.205.3332,333
4*26.721.052113.404.0001,750
B*25.401.00097.273.431,501
5*24.800.97690.723.2001,400
6*23.350.91975.602.6671,166
6.278*23.000.90672.262.5491,114
7*22.180.87364.802.2861,000
8*21.210.83556.702.000875
9*20.390.80350.401.778778
1019.690.77545.361.600700
11*19.070.75141.241.454636
1218.530.72937.801.333583
13*18.040.71034.891.231538
14*17.600.69332.401.143500
15*17.210.67730.241.067467
1616.830.66328.351.000438
17*16.500.65026.680.941412
18*16.190.63725.200.889389
2015.630.61522.680.800350
22*15.130.59620.620.728319
2414.700.57918.900.667292
26*14.310.56417.440.615269
2813.970.55016.200.571250
3213.360.52614.170.500219
36*12.850.50612.590.444194
40*12.400.48811.340.400175
67.6210.410.4106.710.237104

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

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<span class="mw-page-title-main">Caliber</span> Internal diameter of the barrel of a gun

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References

  1. 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.
  2. Fourten Shotgun Resources. (n.d.). http://www.fourten.org.uk/shotgunbores.html
  3. 1 2 Krause, Angel. "What Shotgun Gauge Should I Use for Hunting?". Outly. Retrieved 27 November 2021.
  4. Carter, Greg Lee (2002). Guns in American Society: An Encyclopedia. Santa Barbara, Calif.; Oxford: ABC-CLIO. p. 361. ISBN   978-1-57607-268-4.
  5. 1 2 Frank C. Barnes (2009). Layne Simpson (ed.). Cartridges of the World (12th ed.).
  6. Clair Rees (March 2000). "Marlin's 'Garden Gun'—Model 25MG". Guns Magazine. Archived from the original on 2008-04-29. Retrieved 2008-05-08.
  7. Frank C. Barnes (2003). Stan Skinner (ed.). Cartridges of the World (10th ed.). Krause Publications. ISBN   0-87349-605-1.
  8. 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
  9. Randy Wakeman (2007). "Why the 10 Gauge Shotgun Is Obsolete". chuckhawks.com. Archived from the original on 2006-05-13.
  10. 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. doi:10.1289/ehp.7791. PMC   1257598 . PMID   15929896.