Chief EG-X
From Halopedia, the Halo wiki
Chief EG-X is an Easter egg message that appears on the shell casings ejected by the human based projectile weapons the Battle Rifle, Assault Rifle, Pistol, Sniper Rifle, and Machine Gun Turret. On the base of these cartridges there is writing that says "CHIEF" then "EG-X"; the first word obviously being a reference to John-117. As the writing is small it is difficult to read from even a moderate distance, and because in Halo 3 the bullet casings disappear after a short time, it is easiest to see the writing on the casings in Theater mode, when you are zoomed in (best done when you are detached from the player). The egg appears in Halo 3, Halo 3: ODST, Halo: Reach and Halo 5: Guardians.
