Tracers show the other team where you are. The other team likes to shoot stuff that uses tracers(i.e. crew served heavy machine guns). I do know of some squad leaders who use magazines of tracers in combat to point out enemy positions but everytime you use a tracer you give away your position and that decreases surviability.Why would you not load a full mag of tracers? I always figured the military didn't load full mags of them because they were more expensive than regular ammo. Is there a safety reason behind this?