Specifically, it is a US .31-caliber Colt Revolver bullet, cast in bulletmolds made by the Colt Arms company. It was in use from 1849 (when Colt's .31-caliber revolver, which he called his "Pocket-Model," was first introduced) into the 1870s, after which they fell out of favor due to the advent of metallic-cartridge bullets, which made loading the pistol MUCH simpler.
Your Colt .31 Revolver bullet is a fairly common civil war era bullet, used by both sides in that war. However, none were actually issued by the Ordnance Department of either side's armies... they were used in revolvers which the soldiers purchased themselves, from gun-shops and sutlers. The general rule was, if the Army did not issue the gun, the Army also did not provide ammunition for it. You had to purchase (or cast) your own bullets for it. That is why we dig so many "field-cast" pistol bullets in civil war troop campsites.
