Picture of the Bullet Casing. Note the Halo 3 engine, which only allows the casing to take form of a hexagon shape.
Chief EG-X is an Easter egg that appears in Halo 3, Halo 3: ODST, and Halo: Reach.
Description[]
In Halo 3, all human projectile weapons eject shell casings when firing. 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. This is best done when the camera is detached from the player and zoomed in.
This egg also appears in Halo 3: ODST, despite John-117's absence from the game, as the two use the same engine. They were also present in the Halo: Reach Beta and Halo: Reach itself.