Ever seen a spot like this in a barrel?

[ButterBean]

If it's shooting fine don't worry about.

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[Old Rooster]

I still use Bore Tech to clean copper with but when it's gone I won't buy anymore as Thorrclen has replaced it.
Many members on this forum turned me on to it and it is good stuff if you follow directions to the letter!
They have a video to watch as well.