To date I have in excess of 300+ pure brass C.E.B Safari Raptors down the tube of my .50cal wildcat with absolutely no signs of any metal fouling whatsoever.
Infact (and I'm almost ashamed to admit it), I only just recently cleaned the barrel out for the very first time at the end of the previous hunting season and was shocked by just how little debri was found and how easy, and quick, it was get get clean jags comming back through after only a brief clean.
Weather this can be attributed to the quality of the barrel (Pacnor) I cannot say, but I'm also running the C.E.B brass Raptors exclusively through my Verney Carron double .500 with the same non-issues regarding build-up.
To be perferctly honest, even IF these brass slugs DID leave some fouling I would tolerate it based purely on their extreme performance on game.
Straight line & deep penetration, less pressure, less barrel strain, increased velocities and extreme preformance on game in the field !
Cheers,
Paul.