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.

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)

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

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—	1666 2/3
7	.89	1000
8	.85—	875
9	.81—	777 1/9
10	.79	700
M11P	.77—	636 4/11
12	.75x	583 1/3
13	.74—	538 6/13
M14S	.72—	500
15	.70x	466 2/3
16	.69—	437 1/2
c17P	.67x	411 13/17
18	.66	388 5/9
19	.65x	368 5/19
20	.63x	355
21	.63	333 1/3
22	.62x	318 2/11
23	.61x	304 8/23
24	.61	291 2/3
25	.60x	280
26	.59x	269 3/13
27	.59	269 7/27
28	.58x	250
29	.58—	241 11/29
30	.57	233 1/3
31	.56x	225 25/31
32	.56—	218 3/4

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