Sorry to come in late on this one with my .2 cents. Agreed it does resemble an iron cannon ball more than a mill ball. I bought what I thought was a cannonball a number of years ago, but under the grime, it proved to be a steel mill ball (
).
I think this is a cannon ball, perhaps used as hotshot and deformed on impact. I have seen (hee hee) balls with flattened sides and bent/deformed bar-shot that struck hard structures.
Still another possibility is that this is a cannon ball made using the 'shot tower' method. I've included two sites explaining this innovative method of making cannon shot, and although one confesses that the balls produced were perfectly round, I beg to differ. Confederate cannon balls were often ovoid, had rough seams, were oblong, etc. In any case, a dropped shot could explain the deformity in times of desparation during war-time. I imagine it would still have fired poorly, though...
http://www.waymarking.com/waymarks/W...er_Columbus_OH
http://www.examiner.com/downtown-bal...pplied-physics