Knowing what the metal is would help for identifying the object. The only thing you tell us which might give us a clue about the metal is your guess that the item might be a musketball ...which suggests the metal is lead. Is it lead?
Lacking that info, my preliminary guess (just a guess at this point) is that it is the finial from a bayonet-scabbard's tip or large knife-scabbard's tip. But if your estimate of the ball being "about 1/2-inch" in diameter is accurate, it's unusually large for a scabbard-tip finial ball.