How was bore/caliber size expressed in the period?

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: 

                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     

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

•Modern 21st century "Pound" & 18th century English/American "Pound“1.0 Pound = 453.59 Grams
•Grains in Modern "Pound" = 7000
•French 18th Century "Pound“1.0 Pound = 489 Grams
•Dutch 18th Century "Pound“1.0 Pound = 494.09 Grams
•German (Prussia) 18th Century "Pound“1.0 Pound = 467.4 Grams
•Danish 18th Century "Pound“1.0 Pound = 500 Grams
•Swedish 18th Century "Pound“1.0 Pound = 425.08 Grams

These distinctions in units of measure become especially important when trying to compare firelocks as bore is a function of how many ball to the local "pound" the table above is in "modern" pound equivalents.