This is a curious one. I would almost say it's a carbine bullet based on it's shape, but I can't match those dimensions to anything in M&M, at least not with a solid base. Someone with more field experience will have to make the call on this one.
It is an unfired civil war US .50-caliber Warner Rifle bullet, missing its metal casing. Its base is somewhat corroded AND "concreted" by the blackpowder that was in the unfired casing. It is a rare bullet. See bullet #83 in the updated "Handbook Of Civil war Bullets & Cartridges" by James E. Thomas and Dean S. Thomas.