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Bore diameter from Bore number. A calculation.

How to calculate the theoretical calibre in inches or centimeters of your antique [or modern] weapon which is measured in "Bores"

Of course you know that the Bore is an old fashioned method of sizing the calibre of a barrel. It was calculated by the number of pure lead balls of internal barrel diameter it took to make up exactly one pound of lead. I don't know if the divisions were exact - if so, the original figures must have been calculated by a genius, or maybe they were originally calculated by trial and error??

Calibre in centimeters = 2 times the Cube root of (9.5466 divided by Bore Number). Then divide by 2.54 to convert to inches. Note that the actual calibre of an antique varies a little either way from the theoretical value.

You will need a scientific calculator to do cube root. You have one on your PC. Click "Start". Choose "Accessories", Click "Calculator", Click "View" and choose "Scientific". Enter the result of 9.55 divided by Bore Number (eg 12 or 10 or 24 etc) and tick the "inv" box by clicking on it. Then click on x^3 function in the left pink area and you get the cube root value.

This is the radius so multiply by 2 to get the diameter or calibre in centimeters. Convert to inches - divide by 2.54, and you have the approximate calibre in inches. [If you don't believe me work it out yourself from scratch - I could be wrong!]