Bart B.,
That's an interesting point about soot in post 16. Also, you'd have a hard time spotting an FMJ's open base. It is possible the copper discs I've seen were all from 45 Auto loads propelled by fairly clean-burning powder. So, not too fast and not too dirty. I can't recall if I've picked them up from anything else or not. I just remember the light had to be exactly right and I could spot them at every shot for a short time, and then the sun moved too far to pick them up any longer.