Our very own TheCannonBallGuy has a very informative page, hopefully he'll bless us with his vast knowledge, by answering your post. In the meantime peruse his page for information. A big thing is accurate weight, dimensions, material made from etc. Electrolysis or knocking off of the rust, crud also helps.
At that size it would be a grapeshot. Check the local history and that may tell the tale. Lots of colonial and pirate activity on the coast from your area all the way south to Florida.
Welcome to the Tnet forums from Toronto Ancientpaths.
Nice find and even nicer that you have an I.D. of your cannonball.
Now all you need to do is find a local electrolysis expert in NS to restore/preserve it before it falls to pieces.