Your bullet is one of the early-model (1850s) Colt bullets for revolvers. In the photo, your bullet's diameter appears to be just slightly less than half of a US dime's diameter (.70-inch), so it is probably a .31 early-model Colt bullet. In the "Handbook Of Civil War Bullets & Cartridges" it is shown as bullet #4. In the McKee-&-Mason book on civil war bullets ("Civil War Projectiles II"), it is #505.