Halo 3 - Spartan Scout Shoulder Armour?

[Gallery of suicide...]

I have unlocked the "We're in for some chop" achievement and I have completed the game but I still do not have the Spartan Scout Shoulder Armour... Why hasn't it been unlocked?
I completed the game on easy by the way, however I have completed several levels on other difficulties but the armour is supposed to only need the "We're in for some chop" achievement to unlock it.

 

[B-Rabbit]

You need to find some of the skulls..there are some other online achievements you need to complete to get some of the armour.Here is a list.

http://wiki.wegame.com/Halo_3_Armor/?gclid=CNfGitfRtZQCFQwxiQodLCjVTQ