Have you tried giving it hone with JB’s, I’m no expert but my 6.5-284 was doing that and smith Tony Titheridge got out the JB’s and it made all the difference.
I addition an old fella showed me to use a funnel and tube to pour a jug of boiling water down the barrel before you start cleaning and it really sped up the process. Not sure how that would help copper removal but hey you couldn’t touch the barrel for heat!
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