Progress is slow in Halo 3 mostly as the campaign is difficult to speedrun the first time through.
In the Cortana map where the Master Chief is attempting to recover Cortant from the Flood there is a flame gun on the ground that I stumbled upon while fighting off aggressive attacks.
Earlier there was a removable heavy machine gun. The the Covenant mounted plasma gun could be taken. Now there is a heavy flame weapon.
Progress has been slow in Halo 3 and more screenshots are probably the best course of action.