I've got a relatively new 7 WSM Bartlein barrel on a custom action. The barrel has about 500 rounds on it and has been cleaned regularly. It was never shot hot. The barrel is loosing accuracy fast and nothing seems to help. I took a look through the bore scope and found a jet black carbon ring at the end of the chamber.
I took a bore mop and soaked it with Bore Tech carbon remover and left it sitting against the ring for two days. I refreshed the carbon solvent several times. The ring appeared unchanged. I put a bronze brush on a short rod and tried to spin some of the carbon off thinking that it was probably soft after the soaking. Oddly enough, the black rings still looks about the same.