I am occasionally asked why the British had such odd-number calibers as 7.5 inch and 9.2 inch. The reasons for this date back to the days of black-powder naval cannons, where the guns were rated first in terms of the weight of the gun itself, then by the weight of the shell they fired and only then by the bore size. For example, the "40 ton gun" was the weight of the gun itself and the "12-pdr. 18cwt " had a bore size of 3 inches but was instead designated by the weight of the shell fired followed by the weight of the mounting. The British carried on this tradition for a longer period than did the other major naval powers of the world, who started designating their guns in terms of the diameter (caliber) of the bore by the middle part of the 19th century. For these other nations, designing a larger gun meant increasing the bore size, which of course increased the shell weight, but the British thought first in terms of increasing the shell weight and then designing a cannon to fire that size shell.

When this is taken into account, the reasons for the odd-number size of British guns of the late 19th century become clearer. They are actually based upon the shell weight, not bore size. To state this principle simply, with a few exceptions, British guns during the latter part of the 19th century were designed in a series such that any particular gun in the series would throw a shell twice as heavy as the next-smaller sized gun and half as heavy as the next-larger size gun. To clarify this, examine the major calibers in use by 1900:

Caliber Shell mass (lbs.)
37 mm 1 (later guns: 1.5)
47 mm 3
57 mm 6
3" gun 12
4" gun 25
4.7" 45 (later guns: 50)
6" 100
7.5" 200
9.2" 380
12" 850

Note that the shell weight doubles or nearly doubles for each increase in bore size. Also note that the 4.7" gun was in reality 12 cm (4.724"), one of the few metric-sized guns ever used in the Royal Navy.

This practice of doubling the shell weight for each increase in caliber ended in the early 20th century. Instead, the 13.5" gun fired a 1,250 lbs. shell, or about 1.5 times the weight of the 12" shell. Likewise, the 15" gun fired a 1,920 lbs. shell which was, again, about 1.5 times the weight of the previous caliber.

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7 March 2005