Caliber - Pounds As A Measure of Cannon Bore

Pounds As A Measure of Cannon Bore

Smoothbore cannon and carronade bores are designated by the weight in Imperial pounds of round solid iron shot of diameter to fit the bore. Standard sizes are 6, 12, 18, 24, 32 and 42 pounds, with some 68 pound weapons, and other nonstandard weapons using the same scheme. See Carronade#Ordnance.

From about the middle of the 17th century until the middle of the 19th century, measurement of the bore of large gunpowder weapons was usually expressed as the weight of its iron shot in pounds. Iron shot was used as the standard reference because iron was the most common material used for artillery ammunition during that period and solid spherical shot the most common form encountered. Artillery was classified thereby into standard categories, with 3 pounders (pdr.), 4 pdr., 6 pdr., 8 pdr., 9 pdr, 12 pdr, 18 pdr., 24 pdr. and 32 pdr. being the most common sizes encountered; although larger, smaller and intermediate sizes existed.

However, in practice there was significant variation in the actual mass of the projectile for a given nominal shot weight. The country of manufacture is a significant consideration when determining bore diameters. For example, the French livre, until 1812, had a mass of 489.5 g whilst the contemporary English (Avoirdupois) pound massed approximately 454 g. Thus, a French 32 pdr. at the Battle of Trafalgar threw a shot with 1.138 kg more mass than an English 32 pdr.

Complicating matters further, muzzle loaded weapons require a significant gap between the sides of the tube bore and the surface of the shot. This is necessary so that the projectile may be inserted from the mouth to the base of the tube and seated securely adjacent the propellant charge with relative ease. The gap, called windage, increases the size of the bore with respect to the diameter of the shot somewhere between 10% and 20% depending upon the year the tube was cast and the foundry responsible.

The relationship between bore diameter and projectile weight was severed following the widespread adoption of rifled weapons during the latter part of the 19th century. While guns continued to be classed by the weight of their projectile into the middle of the 20th century, particularly in British service, this value no longer had any relation to the bore diameter as the projectiles themselves were no longer simple spheres and in any case were now more often hollow shells with explosive fillings rather than solid iron shot.