Before I say anything else... let me applaud you for seeking information to correctly identify what you found. Please, never feel embarrassed about doing that. It's better to find the Facts first, than to make an over-eager claim of authenticity, which then gets shot down. THAT is much more embarrassing.
Now... sorry to have to tell you, your item is definitely not a cannonball. All actual cannonballs -- and Grapeshot balls, and Canister balls) were made of metal ...specifically, Cast Iron. Based on your statement of this ball's size (2 inches in diameter) and weight (8.18 ounces), it is not made of metal. According to the US (and CS) Ordnance Maunual, a 2-inch iron ball weighs a bit more than one pound. This ball weighs only about half of that. So, the data confirms what your metal-detector told you.
Some people may wonder, "Well, what if this ball is hollow (a shell or a grenade) instead of a solid-shot?" That is a reasonable question, because a hollow metal ball COULD weigh about half as much as a solid one. But, there was no hollow cannonball (or round grenade) that was 2-inches (or less) in diameter. (The smallest is a Confederate "ball" grenade which is 2-&-1/2-inches in diameter.)
Regards,
Pete [P.C. George]