Not a problem. Also known as brimstone. Sulphur by product of BP combustion.
TC
As I understand it, at certain atmospheric & humidity conditions, maybe both are requited, the "Barrel Blood", red berries will form and be seen at the muzzle.
As far as happening only at a "correct load", that's a interesting observation, due to when it happens in my 14 bore, it happens with any charge, from 82gr. to 165gr., which seems to support the hypothesis of it being a humidity or atmospheric condition phenomenon. Maybe those are all m y best loads - they are only ones I use.