What I've noticed is that when I load plated or coated, I get a lot of leading at the comp. On an open gun, this is not the issue. So the hypothesis would be that in 5 inches the plating or coating is still good, but in a 16" barrel, the plating and coating wears off before leaving the comp, hence leading. I don't shoot PCC much, but all loads would be CMJ just because removing lead out of a comp is a PITA.