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 How
was bore size expressed in the period?
Short
answer is in the number of balls to a pound of lead, which, is the equivalent of
"gauge" as is used on most modern shotguns, the exception being the
.410, which is .41 caliber - go figure. .75 cal = 11 balls/lb, and .69 cal = 15
balls/lb though these are usually rounded off to their modern shotgun equivalents of 12
& 16 gauge, respectively. Though the period way would be to refer to it as a 11 bore rather then a 12 gauge. Some of the more common ones (roughly
rounded off) are:
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Gauge/Balls/lb
Caliber
Weight/gr.
8 .835 875
10 .776 700
12 .730 583
13 .710 538
14 .693 500
16 .663 438
20 .615 350
24 .579 292
28 .550 250
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Knowing
that there are 7000 grains in a pound, we can divide 7000 by 28 and see that 28
gauge balls weigh 250 grains each. If we measure one of those balls, we see that
it is .550 inches in diameter. In modern usage that is the same as '.55
caliber'. The same relationship holds true for any size bore.
While
"Caliber" was a period term,
it was used by artillery as a measure of the relative length of a gun
barrel, defined as the length divided by the diameter of the bore. Thus a
50-caliber gun on a warship has a barrel 50 times longer than its bore.
Confining the shell within the barrel for a longer time increases the velocity,
so guns with a higher caliber usually have a longer range.
I'm not sure when it came to refer to the bore size expressed in
hundredths of an inch, which is what it means today.
At least in England the use of bore versus caliber seems to be the norm
through the 1850’s at least
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