Specifically, it is a .58-caliber civil war Williams "Regulation" bullet, manufactured only by the Yankees. It first shows up on battlefields in 1863, and was used extensively through the end of the war. Therefore, it is a fairly common variety of civil war bullet, and a nice-condition unfired one like yours sells at relic-shows for $4 to $5. I should mention, it is shown the McKee-&-Mason book on civil war bullets as a "pistol-carbine" bullet, but that ID has been proven incorrect. It is definitely one of the varieties of Williams bullets, and it was used in various .577 and .58-caliber firearms.