Marathon (Security) Armor

You unlock the Security Armor in Halo 3 by getting Gamerscore in the game (ie: unlock achievements). To get the Shoulder Armor, you will need to rack up a whopping 700-850 GS (it seems to vary for some reason).
Note: There is a known glitch in the game that will not allow players to unlock the Security Shoulders sometimes.
This might be one of the last pieces of Armor you unlock in the game, as it requires all of the achievements in Halo 3 in order to unlock it. That’s right. You need 1000/1000 GS on Halo 3 to unlock the Security Helmet.