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 Halo 1 that your partner and you can kill one another (one a time) to restart with an initial ammo load when you started the stage.