Halo 2: Hint - Reset Thy Ammo

Expended all your ammo at the end of one stage and don't have enough to muscle past the start of the next one? Save and quit your game -- provided you acutally unlocked a new stage on the Stage Select, you can head back into your campaign (via stage select) and get a fresh batch of pre-assigned weapons for Master Chief.

Co-op players already know from the first game that your partner and you can kill one another (one a time) to restart with an initial ammo load when you started the stage.