I hate to be a downer but I don't think that's a fire misket ball. I've dug over 60 pounds of musket balls and that appears to just a piece of random lead...keep looking though...there are definitely more than enough to go around!
Agreed, not a musket ball. If there is a fort there though there are probably musket balls. Musket balls will only be found where there is some sort of military activity. I have been detecting for forty some years and have found tons of muzzleloading round balls but no musket balls.