Whenever you run out of ammo or are running low on any mission in Solo mode, find a non-essential teammate (someone not involved in the story line) and aim the reticule next to his head or torso. Knife the air and move slightly as you do so toward them so that the reticule is over them. If done correctly, your teammate will die and drop his weapon. The difficulty setting doesn't matter. Non-essential teammates are replaced when you walk into a battle. If the teammate does not die but keeps getting back up, it means they are essential to the story (for example, Sgt. Roebuck and Sgt. Rezenov).