Pistol or squirrel gun, could have been buckshot in a muzzle loading shotgun, true identity will never be known. .36 was a very popular pistol round in the mid 1800s. Definitely not from a musket which is a military long arm with calibers from .54 up to .75. It is possible that it was fired as buck or buck and ball from a musket but most likely a pistol round, nobody will ever know. Having shot a lot of muzzle loaders as a kid we fired all kinds of stuff from them and I am sure people back in the day did the same. BBs, lead pellet gun pellets, marbles, balls of various sizes both lead and steel. Many times more muzzle loading balls have been fired in this country for target practice and hunting than fired in battle. Muzzle loaders were still in use into the late 1800s and even into the early 1920s in some rural areas so they are also impossible to date. Always nice to find something that brings you back to a different time.