Use the following trick if run out or are running low on ammunition on any mission in Solo mode on any difficulty. Find a non-essential teammate (someone not involved in the story line). Walk up to him, and aim the reticule next to his head or torso. Knife the air, and as you do so move slightly towards them so that the reticule is over them. If done correctly, he should die and drop his weapon. Non-essential teammates will get replaced when you walk into a battle. If the teammate does not die but keeps getting back up, they are essential to the story (for example, Sgt. Roebuck and Sgt. Rezenov).