Yeah, try the FHM and see if it does it. They use fully jacketed bullets with no exposed lead at the back kind of like campro's. I know with my PCC I would get massive amounts of lead build up from most factory brands that have open lead at the back because when the powder ignites it flashes the lead and vaporizes it and the pressure pushes it along after the projectile, escaping at the first place open... Like the ports on a comp, or the ports on your barrel.
Or simple answer...wear a protective arm sleeve on your support arm, that should do it.