As Kim and David indicated, I'm here to help folks out on questions like yours.
The bore-diameter of a 6-pounder cannon is 3.67-inches. Because it is a muzzle-loading cannon (loaded from the front end), its projectiles NEEDED to be significantly smaller in diameter than its bore. (Because, if too small a cannonball was used, it would "fall short" when fired ...and if it was too large, it might jam in the barrel upon firing.) So, 6-pounder (caliber) cannonballs were VERY carefully manufactured (and inspected) to always be exactly 3.58-inches in diameter, to "properly" fit the 6-pounder cannon's 3.67-inch bore.
Just a suggestion, for you and other relic-diggers:
PRECISION-measuring is hugely important for identifying/confirming all sorts of relics ...from buckles to bullets to artillery projectiles. So, I recommend buying a set of Precision Calipers, for determining a relic's very-exact size. Harbor Freight Tools sells an excellent set of such Calipers (with an electronic Digital readout) for just $15.95 It's very much worth that small investment of money. Besides helping with relic-identification, owning a good set of precision Calipers can protect you from buying an expensive fake buckle, bullet, or a "not-cannonball."
I'm sorry to have to be "the bearer of bad news" ...but ...the fact that your round iron ball has a hole going all the way through it disqualifies it from being any kind of cannonball. (By the way, a Bar-Shot ball would have a threaded hole in one side, but not go all the way through the ball to the other side.)
I do want to mention, though, that you definitely did the right thing by posting the item for identification/confirmation. Nobody should feel embarrassed about doing that, even if the item turns out to be not-a-relic. It's one of the best ways to learn the Actual Facts about the items we relic-hunters dig up.
Regards,
TheCannonballGuy [PCGeorge ...co-author of "Field Artillery Projectiles of the American Civil War"]