As there are NO "Rules of thumb" that apply to all gauges, take this comment with some common sense:
The "Rule of thumb" for finding a RB to shoot from a smoothbore is to pick a diameter that is .0-20" SMALLER than bore diameter. However, this ASSUMES that you are shooting a Patched round ball using Cotton patches( ala Rifle shooters).
If you are shooting bare lead balls sandwiched between two layers of "tow", or leaves, or some other kind of wad, this rule cannot serve you well, unless the barrel is pretty fouled with BP residue.
If you are paper patching balls, this rule cannot serve you well, unless you find some Darn thick Paper. The paper used in the day is NOT THICK by our standards today, and the Ball diameters were larger than what we would choose to shoot today for BEST ACCURACY. NOw, a lot of the Paper Patched Balls in the day were not much more accurate than bare balls sandwiched between wads of Tow, again, until the barrels were fouled. This was because it was easier to make lead balls a consistent size, than to make barrels that stayed within a narrow range of tolerances for the bore diameters. Brown Bess muskets ran from .760( and some are even larger!)bores, down to .690"( and a few smaller). We can't say if all these dimension were ORIGINAL from the armories, but enough of these old original guns exist in collections in pretty good condition to suggest that bore dimension tolerances were not all that precise.
As a result, the ammo provided for these guns was intentionally cast much smaller. Again, these muskets were intended to POINT AND SHOOT at ranks of enemy soldiers standing a mere 50 yards from the shooter- not pick squirrels out of high trees, with head shots.
This was volley fire, and the survivors then charged each other with fixed bayonets, and fought hand to hand. To reload you had to "negotiate" that bayonet so you didn't STICK YOURSELF, get that powder from the paper "cartridge into the barrel, then stuff the paper patch, and ball into the barrel, and run it down the barrel with your steel ramrod, being careful AGAIN to NOT STICK YOURSELF on your own bayonet. And do it in a HURRY: After all, those enemy soldiers were firing at you, too!
Today, we use tighter fitted balls, and patching, so that these guns can be used to shoot in matches at paper targets. {Its been really difficult finding volunteers to let the shooters fire real balls at, and then, there are those pesky relatives, and the local police and prosecutors who have NO SENSE of humor about these matters, either! :shocked2: :blah: :idunno: :surrender: :thumbsup: :thumbsup: }