Civil war .58-caliber Williams Patent "bore-cleaner" Type 3 bullet, missing the hardened-lead "thumbtack" which fit into the small round hole in the bullet's flat base. See the photo below. The primary ID clues are its round nose and the hole in the base which measures about 1/3rd of the base's width.
The Williams Type 3 bullet's first appearance was in early autumn 1863 (none are found at the Gettysburg battlefield -- July 1863), used through the end of the war, with no further usage known afterward, because the US Army discontinued its muzzleloading rifles shortly after the end of the civil war.