I agree with Mirage83... it appears to be a Confederate imitation of a US .52 Sharps "Old Model" bullet. Like the .520"-diameter Merrill bullet you previously posted, this Sharps bullet's original diameter appears to have been reduced by a century of burial in "wet" acidic ground. The US-made version is shown as bullet #117 in the updated "Handbook Of Civil war Bullets & Cartridges" by James E. Thomas and Dean S. Thomas.