MISCELLANEOUS TECHNICAL ARTICLES BY Dr A R COLLINS

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 (lb), where 1 lb = 0.45 kg.

The values were based of the various cannonball sizes in use, but some simple calculations shed light as to why these particular weights of cannonball came to be chosen.

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{aligned} V &= \frac{4}{3} \pi \left( \frac{d}{2}\right)^3 \\ &= \frac{\pi}{6} d^3 \end{aligned}

If the density of cast iron is $$D$$, the mass $$m$$ of a cast iron sphere of diameter $$d$$ is given by \begin{aligned} m &= DV \\ &= D {\frac{\pi}{6}} d^3 \end{aligned}

The density $$D$$, of cast iron used [1] was 0.2682 lb/in3, therefore \begin{aligned} m &= 0.2682\ {\frac{\pi}{6}} d^3 \\ &= 0.1405\ d^3 \end{aligned} where $$m$$ is in pounds and $$d$$ in inches.

Similarly the diameter $$d$$, of a cast iron ball of mass $$m$$ is given by $$d = 1.9237\ m^{1/3}$$

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

Table 1.  Cast iron sphere mass and corresponding diameter.

From Table 1 it is apparent that many of the Borgard set of round shot weights had diameters very close to whole numbers of inches. This leads to the assumption that the diameter of the cannonballs used in the earlier part of the 17th century were chosen to have a 'nice' numbers of inches dfiameter, 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 somewhat from their original 'nice' values.

16th century gun calibres

The obvious method to verify this theory is to find the calibre of any existing examples of early (pre Borgard) cannon to see if they were indeed 'nice' values. 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 detailed 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 TypeRound 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

Table 2.  Round shot diameter, weight and cannon bore values for various British cannon types taken from the "Gunners Rule" c. 1590 [Museum of Artillery].

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 earlier 16th and 17th century guns was a matter for the skill of the gunfounder. A value of 0.2 to 0.25 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

Table 3.  Round shot diameter, 21/20 cannon calibre and 25/24 cannon calibre for the standard British cannonball weights.

The late 18th century saw the development of a new class of cannon, the Carronade, which fired the standard size cannonballs but had a shorter barrel and lighter weight, making them 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)
(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

Table 4.  Round shot diameter, Carronade bore values for various British cannon sizes c. 1800[4].

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.
4. Henry. C, Napoleonic Naval Armaments 1792-1815 2004.