BRITISH CANNON CALIBRES

Borgard's Standardised Ordnance

At the beginning of the 18th century after George I succeeded to the throne of England. Albert Borgard was appointed to rationalise the Royal Ordnance. He was the first and last person to design a complete system of artillery. His designs were accepted in 1716 but were redesigned when John Armstrong took over as official designer, but the size of the cannonballs was firmly established.

The various dimensions of the cannon were expressed in units of their calibre, the diameter of the round shot they fired. Hence the cannon may be specified to be 18 calibre long, 10 calibre in circumference at the breech, with a 21/20 calibre bore etc. Since the cannonballs were Iron spheres, the weight of a cannon's round shot uniquely determined the barrel bore and gun dimensions. Borgard dispensed with the naming of cannon as Culverin, Minion, Saker etc. and the guns became known by the weight of their round shot. Borgard is credited with standardised British ordnance by standardising on the cannonball sizes with weights of approximately 4lb, 6lb, 9lb, 12lb, 18lb, 24lb, 32lb, and 42lb (where 'lb' denotes pounds, and 1lb = 0.45kg).

Some simple calculations shed light as to why these particular weights of cannonball were chosen as the standard.

Round Shot Diameter vs Mass

Working in Imperial units of inches(in) and pounds(lb) the diameters of the various weight cannonballs can be calculated as follows:

If d is the diameter of a sphere, then the volume V is given by:

$\begin{array}{l} V = \displaystyle\frac{4}{3} \pi \left( \displaystyle\frac{d}{2}\right)^3 \\[9pt] \quad \, = \displaystyle\frac{\pi}{6} d^3 \end{array}$

If the density of Iron is D, the mass m of an Iron sphere of diameter d is given by

$\begin{array}{l} m = DV \\[9pt] \quad \, = D \displaystyle{\frac{\pi}{6}} d^3 \end{array}$

Since the density of Iron D, is 0.2841 lb/in³

$\begin{array}{l} m = 0.2841\ \displaystyle{\frac{\pi}{6}} d^3 \\[9pt] \quad \, = 0.1488\ d^3 \end{array}$

where m is in pounds and d in inches.

Similarly the diameter of an Iron ball of mass $d = 1.8873\ m^ {1/3}$

A table may now be compiled showing the diameter of each of the round shot, if they were exactly 4lb, 6lb and so on, see Table 1.

Mass of Iron Sphere (lb)	Diameter (in)
4	2.99
6	3.43
9	3.93
12	4.32
18	4.95
24	5.44
32	5.99
42	6.56

Table 1. Iron sphere diameter versus mass.

From Table 1 it is apparent that many of the nominal weights of round shot for the standard guns had diameters very close to whole numbers of inches. This leads to the assumption that the diameter of the cannon balls were chosen to be nice round numbers of inches, 3", 4", 5", 6" and the resulting round shot weights happened to be close to the values 4, 9, 18, 32lb. Additional intermediate sizes had weights close to 6, 12 and 24lb, would have been included to give intermediate values to better match the various ship sizes on which the cannon were deployed.

Cannon Bore vs Round Shot Calibre

The bore of the cannon were made larger to allow for the irregularities in manufacture and the fact that the ball diameter increased as the balls rusted. The difference in diameter is termed 'windage'. The windage of early British cannon were specified as 21/20 the round shot diameter, giving a windage value about 5% of the bore. To increase the efficiency of the guns the windage was reduced to 25/24 in the Blomefield pattern guns (1787).

Table 2 shows the mass of Iron balls that have a 'nice' number of inches as their diameter. Also shown is the nominal cannon bore to suit.

Round Shot diameter (in)	Mass of Iron ball (lb)	Cannon Bore (21/20) (in)	Cannon Bore (25/24) (in)
3	4.01	3.15	3.125
3¹/₂	6.38	3.675	3.645
4	9.52	4.2	4.165
4³/₈	12.46	4.594	4.56
5	18.60	5.25	5.20
5¹/₂	24.75	5.775	5.72
6	32.14	6.3	6.25
6⁹/₁₆	42.05	6.89	6.84

Table 2. Round shot diameter and cannon bore for various nominal ball diameters.

There were several revisions to the basic cannon design throughout the 18th century. These changes affected the weight, length, bore etc of the guns so there exist many cannon whose bore may vary by several 10ths of inches but still accept the standard cannonball sizes.

The 'nice' number theory looks reasonable, although the 6⁹/₁₆" calibre would surely have originally been 6¹/₂". Some reason will exist for this anomaly, such as a large inventory of misshapen 6¹/₂" that could only be interchangeable with newer supplies if the newer were manufactured to as larger calibre.

Comparison of Predicted and Specified Bores

The carronade was a short gun developed by the Carron Company, a Scottish ironworks, in 1778. The published Carronade bore values seem to suggest the values derived above may hold some validity.

Nominal mass (lb)	Predicted Bore (in)	Carron Co. Bore (in)
12	4.56	4.52
18	5.20	5.16
24	5.72	5.68
32	6.25	6.25
42	6.84	6.84

Table 3. Comparison of the predicted cannon bore and the Carron Co. carronade values.