What is your definition of "buckshot?"
Most states, and I would say all states have a shot size limit for hunting geese and other game birds. I do not believe that "buckshot" is legal for geese anywhere. But I may be wrong, so check your game regulations.
Buckshot sizes range from the small #4 buckshot that is .24", up to the largest buckshot that is 000 which measures at .36"
Normal goose shot sizes range from no. 2 which is .15" up to T shot which is .20". There is also an F shot, although I have never seen it. F measures .22"
However, will buckshot work in a percussion or flintlock smoothbore? Of course. Buckshot is designed for use in shotguns and smoothbores are basically shotguns regardless of whether they are a muzzleloader or a breechloader.
But then another question comes up. Will a particular smoothbore shoot buckshot with any modicum of accuracy? Only a lot of testing will tell.
