Hello,
I believe that is a fragment from a 9" cannon ball. It certainly looks like a ball fragment but he circumference would need to be determined. You can do this by drawing arcs on some cardboard that would match the bore diameter of balls of that general size. So, for example, since I think that is a 9" ball frag I'd cut an arch of circle that is 8.9" in diameter and then cut it out. The frag will match that curve when you seat it against the cardboard arc. I carry some of these with me as they take guesswork out of ID'ing frags.