Are you near a battlefield? Start by measuring the ball. It should be about .45” (revolver), or 69” (musket ball). Many a union soldier cast balls for their .58 cal rifles once the government issued bullets ran out. I used to collect civil war artifacts, and it looks right…just remember it could be any lead ball from the 1850s to 2000s. I’ve dropped many a lead ball when loading. And, when target shooting, those balls have to end up somewhere. This one may have ended up by where you were walking!
While going to a civil war forum is a good idea, there really is no way of knowing for sure.
Cool find that is very unlikely a rock!