In the days of old, before arms were rifled, missiles were round and shot from a barrel that was smooth bore like a shotgun; the gauge or caliber was numbered according to the quantity of a perfect sphere of lead there was contained in a pound. Thus, 50 gauge rifle was bored for a ball that took fifty of them to weigh a pound, 20 ga. twenty to the pound, 16, 14, 12, 10, etc.
The adjoining table reduces the standard shotgun or smooth bore decimally to thousandths of an inch, also to millimeters.
Gauge Diameter in or bore millimeters.
8 - 21.8
10 - 20.0
12 - 18.6
14 - 17.8
16 - 16.8
20 - 15.6
25 – 14.4
30 - 13.6
32 - 13.2
40 - 12.4
50 - 11.4
Farrow, Edward S. American Small Arms; a Veritable Encyclopedia of Knowledge for Sportsmen and Military Men. New York: Bradford, 1904. Print.
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