That bullet is a civil war era US .44 caliber Colt Revolver bullet. See bullet #34 in the updated "Handbook Of Civil War Bullets & Cartridges" by James E. Thomas and Dean S. Thomas. The book says it was made by the American Powder Company.
(Sidenote: It is extremely similar to the .44 Hazard bullet, but your precise size-measurements prove your is the Colt version. The .44 Hazard is .455"-diameter and .67" long, but your is slightly larger in diameter and longer, proving it is the Colt version.)