As one moves down the calibre graph some of the distinct groupings do seem to correlate with mid-sixteenth-century classes of gun, with the standard base at 28.6 mm (28 mm and 30 mm rounds) and the robinet at 34.9 mm (6 rounds of 34–35 mm). The adjacent calibre of 36–37 mm may represent another group of guns of the same class but perhaps simply of different manufacture. Other peaks do not seem to have a later equivalent, although perhaps the major peak at 56–8 mm relates to a variation on the calibre of the falconet, even though it is later recorded as 51 mm.¹⁴²

It does appear that by 1485 there had already been some degree of standardisation onto a few classes of gun, such as the falcon, which were maintained through the sixteenth century, but perhaps not all the later classes. Either many more classes of small artillery were in use in the late fifteenth century, or manufacturers were producing some guns without clear standards, except perhaps within individual batches. It may be that it was the shift in the technology of barrel manufacture from smithing, using wrought iron, to casting, using bronze or cast iron that finally released the potential for a high level of standardisation. This should be amenable to study, if sufficient lead round shot are recovered from battlefields of the 1450s–1550s because, as discussed below, the firing evidence on some lead round shot shows the type of barrel from which it was fired.

The next question is whether the Bosworth calibre groupings represent multiple firing from a single gun or single rounds from individual guns. Certainly breech loading guns of the period were equipped with several chambers, which could be ready loaded and so enable a number of rounds to be fired in rapid succession. For example, in 1480 the duke of Norfolk had four artillery pieces with 4–6 chambers each.¹⁴³ This question can also be addressed by examining the firing evidence on the Bosworth rounds (Appendix 1). This suggests that of the six 35–36 mm rounds three were fired from composite and one from a solid barrel. The same seems to be true for the six 37–38 mm rounds. In addition, of these 12 rounds at least one was fired from a breech loader and three from muzzle loaders. Thus, while the calibre graph might at first sight suggest an absolute minimum of 10 guns, the firing evidence suggests there were far more. This is also supported by the distribution of calibres across the battlefield (Figure 8.4).

Another possibility is that the grouped rounds came from guns described as ribaudequins or organ guns, which had two or more barrels mounted on a single carriage (Figure 7.31) although, interestingly, just one ribaudequin was listed in the Calais garrison in 1481 amongst a total of 233 cannon.¹⁴⁴ Yet in Henry VIII’s 1513 expedition to France each of the three battles within the army was supplied with 40 organ guns and 60 other artillery pieces.¹⁴⁵ Each barrel of one of these guns might be expected to have had an identical diameter bore, to avoid any confusion of projectiles when loading.