Well, several good photos from all around would be helpful, but you can do these things to see if it's a maybe:
is the metal iron ? if not - i.e. steel - it's not a cannon ball;
carefully weigh it - should be somewhere between 12 and 12.25 pounds (absolutely no more nor much less);
carefully determine the diameter - calipers are best but you can use a flexible measuring tape (e.g. those used for sewing) to get the exact circumference and then divide by pi (3.14159). If it's a cannon ball it should be very close to 4.52 inches. If it's over 4.52 it's not a cannon ball. If it's less it might be but it shouldn't be much less.
The holes are a bit troubling and from the photo I can't really determine what's up with those. Do they have some depth or are they just surface defects ? Are there any other surface markings - i.e. mould marks (should be circular) ?