There are several lines of reasoning that can be used. First, there WERE some standard sizes, but they went by ball per pound or gauge(bore). Daniel Boone's Ticklicker fired an ounce ball according to the history passed down to us. This would have been in the 1770 time frame. That equates to a .60-something caliber ball (437 gr) and would have been called a 16 bore. Battlefield finds also give us a range of calibers: the big bored musket balls, small buckshot for the buck and ball loads and rilfe balls, typically near .50. Yes, there were .40s made back in 1770, but alot of the rifles were bigger bored then. There were still buffalo and elk in the east, not to mention open hostilities with the natives (and soon with the mother country). I personally chose a .50 to represent the period in my own rifle, but it could have been one of the even 'balls to the pound' numbers close to it and been accurate. There were several peaks on my bore size distribution and the most prominant were near .43-.45 and near .48-.52. If you look at rifles used later, say 1800, you will find many fall near the smaller peak size. If I could identify only pre 1770 rifles I am sure they would average slightly above the .48 overall average--I need to try that...