You have a .30-'06 cartridge case manufacutured in 1918. This was the standard rifle cartridge for the U.S. in both world wars. At the opening of the second world war, the U.S. still had many millions of rounds left over from the first world war. Weapons using this ammo include the 1903 Springfield rifle, M1917 Enfield rifle, M1 Garand, BAR, Johnson M1941 rifle, and several machine guns.