How heavy is the cannonball? Looks like a case shot meant to be fired from a 6 pounder. Since yours is hollow (meant to hold gunpowder), it will weigh less than a solid (6 lb) shot. That copper ring on the ball is the fuse holder. Goggle "Civil War case shot" and I am sure that you will find plenty of info. Nice find!
It is a CS ball (can tell by the fuse) most likely a sideloader (look for a lead plug). Without the exact size, it is tough to tell from the pic whether it is a 6lb or 12lb ball. Though again with that fuse (read paper fuse adapter) it is more than likely a 12lb ball.