Merle,
You might want to take a look throught this website, http://www.civilwarartillery.com/ although this is primarily a Civil War artillery site, you might be able to verify that the ball could be a real cannonball.

There are an amazing amount of cast iron balls which are thought to be cannonballs, but are not. Generally-if there is a visable seam where the two halves of a mold are joined in the iron, then chances are that the ball is not a cannonball. If there is a slight offset between the halves- then it is doubtful that the ball was ever intended to be fired. In order to shoot accuratly, the ball had to be free of raised seams, and as perfectly round as possible. all arsenal produced ammunition had to pass through various gauges before being issued.

Oddly shaped cannonballs could get wedged in the bore and create a dangerous chance of bursting a tube. If it did
exit the bore, then it wouldn't fly true if it wasn't round.

If you can post a digital photo of this object that you found, then someone might be able to help you verify what you have.