shotguns

drawing showing relative sizes of some gauges

The gauge of a shotgun was originally the number of round balls just big enough to fit the gun's bore that could be cast from 1 pound of lead. Thus 12 lead balls that fit a twelve-gauge shotgun would weigh 1 pound. Cannons were similarly sized, but this definition was formalized for shotguns by the Gun Barrel Proof Act of 1868 in Great Britain.¹

These hypothetical balls should not be confused with “pumpkin balls,” formerly used with modern guns and now replaced by rifled slugs. Pumpkin balls had to be smaller than the gun's bore in order to get past the choke in modern barrels.

In addition to the gauges shown in the table below, 11-, 13-, 14-, 15- and 19-gauge shotguns have been made at one time or another in the United States, though they no longer are, and 14-, 24-, and 32-gauge guns are still manufactured in Europe. The bore diameters given below are nominal; the actual size of the bore varies from maker to maker.

Gauge Diameter in inches
4 0.935 (cartridges of this size were sold within living memory!)
6 0.919
8 0.835 (became illegal for waterfowl just before World War II)
10 0.775
12 0.729
16 0.662
20 0.615
28 0.550
410 0.410 (actually a calibre, not a gauge)

1. “This happy circumstance is the result of the English Gun Barrel Proof Act of 1868 whereby the London Gunmakers Company and the Guardians of the Birmingham Proof House established dimensional and ballistic standards that have remained in force to this day.” Paul S. Foster, Superintendent, Ammunition Quality Control, Winchester-Western Division, Olin Mathieson Chemical Corporation, in a private communication. Quoted in Ralph A. Sherman.
Can we live in Babel?
in Carl F. Kayan, editor.
Systems of Units. National and International Aspects.
Publication No. 57 of the AAAS.
Washington, D. C.: American Association for the Advancement of Science, 1959.
Page 231.

A pdf of the 1868 Act can be downloaded here. Its table of bores and ball diameters can be found on page 1634.

Cartridges and loads

All American shotguns are now chambered for 2¾ shells. In Europe, 65 mm (2½″) cartridges are standard.

Cartridges of the type referred to as scatter, spreader or brush loads incorporate cardboard disks that result in a wider pattern.

Rifled slugs
Gauge Weight in grains
12 415
16 350
20 282
410 93

sources

1

In rifles and fowling pieces, the diameters of the bores, designated as No. 1. 2, 3. 4. 5. &c., are the diameters respectively of leaden bullets or spheres, of which 1. 2. 3. 4. 5. &c. weigh exactly one pound avoirdupoise; and as the subject may have an interest for some of the readers of this volume, the following particulars of the weights of the balls in grains, and of the diameters both of the balls and of the barrels in hundredths of an inch, are transcribed from Mr. Wilkinson's gage, which he has constructed with great care.

Number Diameter of bore
in Hundreths
Weight of leaden bullet
in Grains.
5 .98 1400
6 .93— 1666²⁄₃
7 .89 1000
8 .85— 875
9 .81— 777¹⁄₉
10 .79 700
M11P .77— 636⁴⁄₁₁
12 .75x 583¹⁄₃
13 .74— 538⁶⁄₁₃
M14S .72— 500
15 .70x 466²⁄₃
16 .69— 437½
c17P .67x 411¹³⁄₁₇
18 .66 388⁵⁄₉
19 .65x 368⁵⁄₁₉
20 .63x 355
21 .63 333¹⁄₃
22 .62x 318²⁄₁₁
23 .61x 304⁸⁄₂₃
24 .61 291²⁄₃
25 .60x 280
26 .59x 269³⁄₁₃
27 .59 269⁷⁄₂₇
28 .58x 250
29 .58— 241¹¹⁄₂₉
30 .57 233¹⁄₃
31 .56x 225²⁵⁄₃₁
32 .56— 218¾

Charles Holtzapffel.
Turning and Mechanical Manipulation..on the Lathe.. Vol. 2.
London: Holtzapffel and Co., 1856.
page 1017.

2

Bore & Ball Diameters
Act of 1868
Gauge Bore
diameter
inches
Ball
diameter
inches
1 1.669 1.649
2 1.325 1.305
3 1.157 1.137
4 1.052 1.032
5 0.976 0.956
6 0.919 0.899
7 0.873 0.853
8 0.835 0.815
9 0.803 0.783
10 0.775 0.755
11 0.751 0.731
12 0.729 0.709
13 0.710 0.690
14 0.693 0.673
15 0.677 0.657
16 0.662 0.642
17 0.649 0.629
18 0.637 0.617
19 0.626 0.606
20 0.615 0.595
21 0.605 0.585
22 0.596 0.576
23 0.587 0.567
24 0.579 0.559
25 0.571 0.551
26 0.563 0.543
27 0.556 0.536
28 0.550 0.530
29 0.543 0.523
30 0.537 0.517
31 0.531 0.511
32 0.526 0.506
33 0.520 0.500
34 0.515 0.495
35 0.510 0.490
36 0.506 0.486
37 0.501 0.481
38 0.497 0.477
39 0.492 0.472
40 0.488 0.468
41 0.484 0.464
42 0.480 0.460
43 0.476 0.456
44 0.473 0.453
45 0.469 0.449
46 0.466 0.446
47 0.463 0.443
48 0.459 0.439
49 0.456 0.436
50 0.453 0.733

The Act also designates gauges A to P, excluding G, I and N, larger than all but 1 and 2 gauge. A gauge, for example, has a bore diameter of 2 inches.

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