The problem is that nominal gauges and calibers are not necessarily standardized. I've probably seen specifics on 20ga most often, because it is so popular nowadays. I can recall seeing references to nominally-20-ga barrels (nominally .615") of modern make with bores that ranged from .610" (or a hair under) up to at least .626". Add a bit of pitting (harder to seal over but with higher friction) or honing (the bore is larger and with lower friction) and the possibilities compound.
Just as one must suit one's ball and patch combination to the individual rifle's bore dimensions, one must suit one's wad diameter and/or combination to the individual fowler's (or musket's, or fusil's, ...) bore.
Regards,
Joel