The British Brown Bess was .75-cal round ball, 17.5mm, so I would put my money on it.
Shotgun buckshot goes to triple-aught #000 Lead Buckshot .36" (9.1mm) then up to 15.2mm tri balls, so right away you are larger than any standard shot size.
Solids/Slugs are relatively modern in terms of shotgun cartridges and tend to look similar to a Mini ball, with spiral fluting/ribbing to make them spin, even wad slugs are more traditionally bullet shaped than ball-shaped. Brenneke slugs were designed in 1898 in Germany and developed to American Foster slugs in 1931......fairly recent history for the shotgun, up until then it was all birdshot and buckshot. In the 1600's the Blunderbuss was designed in Germany then it transformed into a "Fowling gun" or "fowling piece" in the 1700's used in Britain to hunt game birds. Caliber sizes back then would not have been standard, more along the lines of if it fits down the barrel, pack it and let's see. It was not until 1776 in Kentucky the term "Shotgun" was first coined, to differentiate between a smoothbore shotgun and a rifled musket.
https://www.prospectingaustralia.co...26947528_eng_pl_brown-bess-carbine-1219_1.jpg
