BRITISH CANNONBALL SIZES

Borgard's Standardised Ordnance

In 1712, Colonel Albert Borgard was appointed Chief Firemaster and later, in 1718, Assistant Surveyor of Ordnance. He rationalized the multitude of gun types then in the Royal Ordnance and specified a complete system of artillery. His designs were accepted in 1716. Few of his gun designs were successful and they were redesigned when Armstrong was appointed Surveyor of Ordnance in 1722. But Borgard's effort to standardize the Ordnance had long lasting effect.

Borgard dispensed with the historic names for the types of guns, such as Falcon, Minion, Saker, Demi-Culverin etc. and specified each type by the weight of the round shot (cannonball) they fired. He further limited the number of cannonball weights to a strict set of values:

The large gun values were:  4, 6, 9, 12, 18, 24, 32, and 42 pounds. (where 1 pound (lb) = 0.45kg).

The values were a subset of the various cannonball weights in use, but some simple calculations shed light as to why these particular weights of cannonball came to be used historically.

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:

If the density of cast iron is D, the mass m of a cast iron sphere of diameter d is given by

The density of cast iron used^[1] was D, is 0.2682 lb/in³

where m is in pounds and d in inches.

Similarly the diameter of an Iron ball of mass

Table 1 shows the diameter of each of the round shot sizes with weights specified by Borgard: 4lb, 6lb and so on.

Mass of Cast Iron ball (lb)	Diameter (in)
4	3.05
6	3.49
9	4.00
12	4.40
18	5.04
24	5.55
32	6.10
42	6.68

From Table 1 it is apparent that most of the Borgard set of weights of round shot had diameters very close to whole numbers of inches. This leads to the assumption that the diameter of the cannon balls were originally chosen to be 'nice' round numbers of inches, 3", 3½", 4", 4½", 5" etc. and the resulting round shot weights happened to be close to the values 4, 6, 9, 12, 18 lb. Borgard just rounded the weights to the nearest pound, meaning the shot diameters varied some what from their original 'nice' values.

16th Century Gun Callers

The obvious method to verify this theory is to find the calibre of any existing examples of early (pre Brogard) cannon. The Museum of Artillery - Woolwich has several examples, but it also holds a "Gunners Rule" dated at c. 1590. On it is written a table of shot weight, powder charges etc to be used for the various guns in use during that era: Saker, Culverin, ... etc. Clearly there was no tight standard for these gun types, as the table shows values for small, average and large versions of most types. The detail dimensions of the gun were left to the skill of the gun founder. Boring a long smooth barrel was difficult and the final calibre of the guns must have varied a great deal. Hence the need for gunners to have a ready reference to know the appropriate shot and powder charge to use once he had measured the calibre of his gun.

Table 2 shows values taken from the table on the "Gunner Rule" giving the round shot diameter, weight and the calibre for each of the early gun types. The shot sizes that Albert Borgard appears to have used as the basis of his standard set are highlighted in yellow.

Gun Type	Round Shot diameter (in)	Shot Weight (lb)	Cannon Bore (in)
Minion	27/8	31/4	3
	3	33/4	31/4
Saker	31/4	43/4	31/2
	31/2	6	33/4
	33/4	71/2	4
Demi Culverin	4	9	41/4
	41/4	103/4	41/2
	41/2	123/4	43/4
Culverin	43/4	15	5
	5	171/2	51/4
	51/4	20	51/2
Demi Cannon	6	30	61/4
	61/8	32	61/2
	63/8	36	63/4

Cannon Calibre and Windage

The bore of the cannon were made larger to allow for the rough casting and rusting of the cannonballs, as well as the irregularities in the casting and boring of the gun. The difference in diameter is termed 'windage'. The final bore diameter and hence the windage of 16th century guns was a matter for the skill of the gun founder. A value of 0.2 to .25 inches was typical.

Albert Borgard specified that along with the weight of cannonballs the windage of the guns should be standardised. He specified that the bore diameter should be ²¹/₂₀ of the gun's round shot diameter. This gives a windage value of 0.2" for a 4 lb cannon but gives a rather large 0.33" for a 42 pounder. To increase the efficiency of the guns the windage was reduced to ²⁵/₂₄ in the Blomefield pattern guns (1787).

Table 3 shows the cannon calibre values for both these windage standards.

Round Shot Weight (lb)	Round Shot Diameter (in)	21/20 Calibre (in)	25/24 Calibre (in)
4	3.05	3.20	3.18
6	3.50	3.67	3.65
9	4.00	4.20	4.17
12	4.40	4.62	4.58
18	5.04	5.29	5.25
24	5.55	5.82	5.78
32	6.10	6.41	6.36
42	6.68	7.02	6.96

Carronade Windage

The late 18th century saw the development of a new class of cannon firing the standard cannonballs but with a shorter barrel and lighter weight, they were much easier to handle aboard ship. Their shorter barrel meant that they could be bored more accurately. As a result the Carron Co. that produced them, was able to reduce the windage by using a bore ³⁵/₃₄ of the shot diameter.

Table 4 shows the Carronade calibre values^[3] for the various cannonball sizes.

Round Shot Weight (lb)	Round Shot Diameter (in)	Carronade Bore 35/34 (in)
12	4.40	4.52
18	5.04	5.16
24	5.55	5.68
32	6.10	6.25
42	6.68	6.84

References:
1. Muller. J, A Treatise of Artillery 1768.
2. LeFroy. J. H, Official Catalogue of Museum of Artillery -Woolwich 1864.
3. Douglas. H, A Treatise on Naval Gunnery 1829.