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STC my first thought aswell, but if it was rust a simple clean would have made a difference to it but I tried multiple things for 3 nights until it started to make any difference, it was some sort of...

Looks like it might be the remains of a similar coating, that is slowly wearing away.

Some pictures to give you an idea of what was in mine
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I had a similar experience, a brand new bore I had had images like that and also a brown sort of looking stuff that had me panicking thinking it was rust, this was in a brand new unfired barrel,...