If run out or are running low on ammo on any mission in Solo mode, find a non-essential teammate (someone not involved in the story line). Walk up to him and aim the reticule near 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, your teammate should die and should drop his weapon. The difficulty setting doesn't matter. 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).