"Head: Get a gamerscore of at least 1000 in Halo 3. Since the new achievements from Title Update 2, the head may be unlocked from around 40-50 non-vidmaster achievements.
Shoulders: Officially, the Security shoulders unlock when the player has attained at least 750 Gamerscore points from Halo 3. However, the shoulders are renowned and well-known bugged items. There have been inclusive reports of players unlocking the shoulders at Gamerscores from the 300s to 1000. There has also been reports of a glitch in which the Security shoulders will not be unlocked until 850 or 900 gamerscore and will re-lock themselves for no apparent reason, or the shoulders being unlocked at the required 750 Gamerscore, then having them lock again, only to unlock when the helmet is attained with a Gamerscore of 1000. The Security shoulders may glitch into appearance after unlocking all 9 gold skulls sequentially. A perfect 1000 Gamerscore has been reported to guarantee unlocking the Security shoulders permanently. Some have reported unlocking them at only 30+ achievements."