an anecdotal addition.
.662" is 16 bore, .678" is 15 bore & 14 bore being .693".

To be sighted to 2,400 'paces' means that gun in the picture was always designed for the conical 'ball' as a round ball that size will not traverse that distance in the air - it is virtually impossible. For a 'conical' of that date to be accurate at that range when from a military rifle, is equally impossible. Perhaps a volley fire into a huge encampment could possibly hit someone or perhaps an unlucky horse.

In around the 1850's when the .69 calibre American muskets were rifled (following the development of this system in Europe ie: 1840's), the US military used a 730gr. conical with a mere 70gr. musket powder for about 900fps - in the .69's - ie; rifled muskets. The .54's and .58's were not rifled muskets as they had never been produced as muskets, but always as rifles.

I do not have knowledge of the size or weights of the various 'conical' 'balls' used on Continental Europe. But do know they used boxwood, wax (or sometimes iron is mentioned) plugs in the hollow 'conical' bases to effectively expand the undersized conical out into the rifling.

In 'America', the skirt was thinned so the powder charge itself would expand the skirts. It was discovered that in the Euro style 'ball', that at times the 'plug' was punched through the ball, which left a ring of lead stuck in the bore, thus the invention of the thin skirted "Minnie Ball".