The firing evidence on the round shot, which is discussed below, shows that that all the rounds were fired from single barrels, rather than as hail-shot from a very large bore gun. Thus the calibre of the shot does indicate the bore of guns in use at Bosworth. Analysis of the Mary Rose assemblage has attempted to link the calibre of projectiles to particular classes of guns retrieved from the wreck or detailed in the records for the ship. With battlefield assemblages such direct comparative study may never be practicable for the late fifteenth century. However, one can look to the broader records for artillery and projectiles available for use by field armies in England at the time, together with the evidence from later military manuals, also taking account of studies like that for the Mary Rose. But there are various problems. The sixteenth-century named gun types may not be applicable to the later fifteenth century, while substantial differences may exist between guns used in a naval as opposed to a battlefield context. For example, the small bore guns known as bases that were in use on the Mary Rose were swivel guns, which is not a type likely to be used on the battlefield.

The calibre of the Bosworth rounds runs contrary to the ideal for battlefield use expressed by Machiavelli who, in 1521, published the earliest of renaissance military manuals. His choice was the saker, which was also the ideal in the seventeenth century, when it was the largest mobile field piece attached to infantry battalions.¹³⁸ In the sixteenth century a standard saker fired a ball of 89 mm according to Burton in c. 1550, although the 1547 list quoted above gives both 76.2 mm and 82.5 mm. The nearest Bosworth round is that of 97 mm, but this is at the very bottom of the range of calibres for the demi-culverin, which in 1547 varied from c. 95.2 mm to 101.6 mm.¹³⁹ Later the demi-culverin was a siege gun that seems only to have been used on the battlefield in fixed batteries, because of its weight.

The next largest round is of 63 mm, which matches Burton’s falcon shot of 63.5 mm. In around 1495 the falcon was by far the commonest artillery piece in the magazine at the Tower of London, where 31 were specifically described as ready for field use. Then, in 1496–7, Henry VII ordered a further 28 falcons to be made from Wealden iron.¹⁴⁰

If the falcon was such a significant field gun just a decade later, why is it represented by just one round at Bosworth? It is possible that the purchases by Henry VII after 1485 represent a new phase in the development of English field artillery, but alternative explanations must be considered. On the Mary Rose the lead and lead composite round shot extended up to 85 mm but concentrated below 50 mm; cast iron rounds concentrated between 55 mm and 195 mm and stone at 135–235 mm.¹⁴¹ Thus the majority of larger rounds at Bosworth might have been mainly of iron or, more likely, of stone. Alternatively, as noted above, the largest rounds may be grossly under-represented because they were far more easily recovered after the battle. However, the 63 mm round does lie on the south-western edge of the round shot scatter. As the largest rounds are likely to have had the longest final range so, if fired by Richard from fixed gun batteries, all the other large rounds may lie beyond the north-western boundary of the survey. Moreover, although the 97 mm round lies in the centre of the round shot scatter, it was the only example found partly buried in the subsoil, which may indicated that it embedded on first bounce and that other large rounds ended up much further to the west after further bounces.