As mentioned, additional detailed images, accurate measurements and weight will help compare the object to known artillery ordnance tables for what could be possible. Particular attention to any noted markings, mold seams, depressions or orifices Just from what can be seen in this image it does appear a bit small for majority of Civil War era ordnance... However Charleston region also had lots of both Rev War and War of 1812 activity amongst others as well.. Many folks would frequently pick up souvenirs and take them home... Hence may or may not be related to a given action where it was discovered.