Devils advocate here: OP states shooting a JSB 18 well over 1000 FPS .... Barrel an obvious POLY profile.
What i'm seeing ( Due to consistency of image texture of bore ) Is a case of EXTREME LEAD FOULING.
I would be more incline to play along after seeing a through Bore clean & scrub with a bronze brush etc ... Picture
Interesting!!! I have never seen a barrel THAT bad with lead fouling, not saying it’s impossible. Even when dirty and not shooting well my poly barrel still looked mirror smooth minus tiny imperfections/lead specs. Certainly would be very interesting to see if a good cleaning would restore it to a mirror finish, certainly won’t cost much to try.
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