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You need abrasives to get a decent change. It's pitted shooting a 4 inch group at 40 meters. it's fucked anyway, it won't get worse from a little scrubbing. And if it does you loose a 4 inch group at...

It sounds fucked, so scrub with scotch bright patches, or even fire lap. Can't do any harm at this stage. If that doesn't work re-barrel. You could re-crown but probably just waisting money.