Here at Veteran Arms LLC, we often get questions about the bore diameters of our muskets - specifically 68 vs 69 caliber for the French muskets and 74 vs 75 caliber for British muskets as well as appropriate ball diameters.
During the bicentennial of the American Revolution, several companies began reproducing replica muskets. The caliber settled upon for the reproduction British muskets was 75 caliber - or about middle range for original pieces, and 69 for French pieces. These supposed "correct" calibers stuck and have been oft repeated almost to the point that they are accepted as the only correct bore sizes. The truth is that forging and machining processes were not nearly as accurate during the 18th and early 19th century and the weapons produced during that period were designed to fall within an acceptable tolerance range.
For the British Brown Bess (and all other weapons with what was called "musket bore") the piece was to have a bore diameter of from .73-.79 inches. In fact, most original pieces existing today have bore diameters from .77-.78 inches. These wide tolerances didn't matter because the balls issued for the piece were significantly smaller than the bore diameters with the median being about .693. Even so, each solder was to test his cartridges upon issue to see whether the balls issued would slip easily into the bore of his musket while still wrapped in their papers and, if not, to either swap them with another man or set them aside to be hammered out of round enough to pass or broken and recast.
The same was true of French muskets like the Charleville. Again, the caliber settled upon approximately 40 years ago was a median caliber - i.e. .69 for reproduction muskets. However, the French government during the 18th century specified the acceptable bore tolerances for muskets to be from .666-.711 inches and for balls for those muskets to have a size of 18-20 "Balles a la livre" or .629-.652 diameter with most existing examples being about .643. Ditto the 1795 Springfield musket which was a copy of the French Charleville Musket.
The bottom line is that 74 caliber is a historically accurate bore diameter for a Brown Bess, and 68 caliber is a historically correct bore diameter for a French weapon of “musket bore” - ie a Charleville or similar American weapon like the 1795 Springfield.