Being a civil war relic (and bullet) digger for nearly 40 years, I can tell with certainty that your bullet's lead body has been significantly corroded by acidic soil. But I recognize it as a civil war era .50-caliber Smith Carbine bullet. (It is strictly a yankee-made bullet, and was used primarily by the yankee Cavalry, not the Infantry.) Your bullet is now a bit smaller than its original diameter due to the acidic soil corrosion. Here's a photo of an un-corroded one.