Absolutely a post-civil-war bullet. As a relic digger myself, I can understand why your friend would very much prefer to think it is a civil war bullet. But if he is so sure about that, ask him to show you that exact version of bullet (including the 4 body-grooves and the crimp-mark from a metal cartridge-casing) in ANY of the books on civil war bullets. I guarantee you he won't be able to do that... because it is definitely a post-civil-war bullet. I'm sorry to have to disappoint him, but wouln't he rather know the truth about it. Please measure its diameter (in hundredths-of-an-inch) with a Digital Caliper, and show us whether its base is flat or has a cavity in it. With that information, we might be able to give you its specific identification.