I found one about the same size at a lost Seminole War fort site up in North Florida... it's about that same size.
I believe it's actually a pistol shot, opposed to a musket shot.
If there are any signs of course threads in the hole in the ball and if it is a round musket ball, then this ball could have been pulled with a ball puller and not fired. Pulling a ball was some times necessary to change shot or to reload with a fresh powder charge. Of course, if you forgot to pour the charge down the bore before loading the ball, then you usually had no option but to pull the ball with a puller. MHO.