I'm working with a new 6.5x284 rifle. Rifle has a Lilja SS barrel with a Vais brake. I always remove the brake and soak it in SC/Kroil while cleaning the barrel. The face of the barrel, which is exposed without the brake in place, is fouled with a black carbon like substance that seems impervious to the SC/Kroil. A stiff plastic brushing with solvent does nothing. Anyone else have this problem? How would you suggest removing it without damaging the crown? If it makes any difference, I'm shooting 49gr H4350 and clean every 20-25 rounds.