Congrats on the find! It does appear that it could be a Cannonball but the exact weight and dimensions are needed to tell for sure. Be careful with it until you can determine whether it is a solid shot or it has black powder inside with a fuse. Clean it up carefully, take pictures all around the ball and post them here and especially if you cannot tell if it has a fuse or not. The fuse will be inside a round hole on the ball and will be flush or nearly flush with the outside. If it is a Cannonball with a fuse and probably black powder inside, just note that professionals can make it inert, so no need to call the Police because they will destroy the ball. Get the exact weight and measurements of the ball. If you have no way to measure the diameter, then measure the circumference of the ball and do the math to get the diameter or post the circumference measurement here and someone will determine the diameter of the ball. I hope it is a Cannonball for you but be safe no matter what!