Musket ball is somewhat of a misnomer. Muskets were originally military arms, and your ball is way smaller than those used in muskets. Most folks fail to realize that there were probably as many round balls fired in non military use as in military use. Target practice, hunting, etc. The round balls were used in pistols, smoothbore long arms, shotguns, rifles, and even slingshots. They came in a plethora of diameters as many early guns were made here and in Europe in many different calibers. Muzzle loading guns were used well into the late 1800s by folks who couldn't afford the more modern guns and their more expensive ammo. There was a re-birth of muzzle loading hunting and shooting started in the 1960s and it has grown ever since. Lead grows a white oxide on it's surface with time and exposure to the elements. This oxide coating then seals the lead and it will no longer oxidize further. Hence, it is almost impossible to tell the difference between one that has been in the ground for 75-100 years from one that has been in the ground for 200 years or more. Even finding one near or on a known battle area doesn't mean it was necessarily used by troops since pretty much all battlefields were hunted in before and after the battle. As military weapons became outdated they were sold off as surplus and often used as is or lightened down for hunting guns. You have an old round ball, but any attempt at positive ID of it's history will be simply conjecture.