?? " In order to have the capability of shooting round balls "
?
Just for the record, I hunt ducks and geese with my 10 bore Chambers New England fowling piece all the time. It is my gun of choice and has brought many fine meals to the table. I also use round balls in the same gun for hunting deer and bear. So far 2 deer and 1 black bear have been put in the freezer with thanks to this gun. And turkey !! So, if you are building a hunting gun, then I suggest that bigger is better.
If you mainly want to shoot at targets, a smaller bore will be easier on the shoulder than the larger, but....that is also up for debate , considering the loads that some shoot. The same shot load in the larger bore as the smaller will give a shorter string and probably pattern better. I have a 20 bore Chambers English fowler which is a fabulous gun, mainly dedicated to partridge. It is faster to handle than the New England. However, if I could have only one, the N.E. Fowler would be it, hands down.
With shot or round balls it is an absolute dependable game getter.