Chief EG-X
From Halopedia, the Halo wiki
File:L 46490758bebe86a15a660ab90633e3dc.jpg
Picture of the Bullet Casing.
In Halo 3, the human based projectile weapons the Battle Rifle, Assault Rifle, Pistol, Sniper Rifle, and Machine Gun Turret, shell casings are ejected when bullets are fired. 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). This egg also appears in Halo 3:ODST, despite the Chief being absent from the game.
