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With the heavy charges of slow-burning serpentine powder first used, guns had to be long to ensure the propellant charge was all burnt before the shot left the bore. Even then variations in loading conditions and in the quality of the powder caused much of it to be wasted. But long guns were not only difficult to load, they were also heavy. At the start of the era, field and siege artillery were coming into their own with a demand for mobility, for guns which could be easily handled, and which required a minimum number of men and horses to move them. As a result many experiments were carried out; we find for example that a 5.5-in culverin of 28 calibres length in Table 1 by 1646 had been shortened to 22 calibres.
A typical experiment carried out in the early 1700s involved shortening a gun by sawing off a calibre's length at a time while progressively reducing the propellant charge, testing the result by firing the gun at a bank of earth, and measuring the distance penetrated by the shot. Thus we find in 1760 the successor to the 5.5-in culverin, the 5.824-in 24-pr to be 19 calibres in length and achieving a satisfactory penetration with eight pounds of corned powder.
But knowledge gained from tests took many years to be put into effect because pieces of ordnance, especially in the Colonies, were expected to last anything up to a century. In New Zealand we had SBML 24-prs made in 1813-14 on coast defence stations up to at least 1893! Note that the design length of a SBML gun was the distance from the rear of the base ring to the face of the muzzle, not the overall length.
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WL Ruffell Issue 88 December 1995 |
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