Halo cheat codes:
In multiplayer, on Gephyrophobia, if you take a banshee, and fall
to the floor, take out a fuel rod gun and shoot downwards just as
you get out of the banshee. If done correctly, you should unlock a
cheat profile, and from there, just play the cheat level after
'the maw' to get cheats. THIS IS SO UNOFFICIAL!!!
type in 11mar and you get 11marines and they have shotguns and full
ammo with the shotgun.