I would call it a round ball, musket infers military use and it and there wasn't really any military action in your area. There are thousands and thousands of these things in the ground, many more were used for hunting and target use than used in all the battles ever fought on our soil. They are also fairly impossible to accurately date since they are still used today, and they oxidize at different rates in different soils. So it could be 100, 200, or fifty years old no way to tell. It looks unfired as in dropped or perhaps missed it's target and all trees since it is still very round. Keep hunting, the cool stuff will turn up.