Re: Barrel Question
It is stubborn copper. Surprisingly, some of the best handlapped bores still have a problem with it. My .277 Hart bbl was the most difficult for me to clean. It took a LOT of work. Try various copper cleaners, as mentioned, the Barnes CR-10 is a good choice, soak the bore awhile but stay within recommendations.
Montana X-treme 50BMG made the biggest difference for me in my Hart bbl., even over Wipeout.
What the barrel makers will tell you though, is that if it isn't hurting accuracy, don't worry about it.