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!)|
|8||0.835 (became illegal for waterfowl just before World War II)|
|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.
A pdf of the 1868 Act can be downloaded here. Its table of bores and ball diameters can be found on page 1634.
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.
|Gauge||Weight in grains|
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
|Weight of leaden bullet
Turning and Mechanical Manipulation..on the Lathe.. Vol. 2.
London: Holtzapffel and Co., 1856.
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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Last revised: 23 June 2016.