My 590A1 had the exact same brown "something" in the chamber when bought new last week. A bore snake followed by a couple hundred rounds of birdshot/buckshot/slugs made no difference.
I chucked a couple of sections of cleaning rod and a fresh 12 gauge brass bore brush in an electric drill, used a generous amount of WD-40, and 2or 3 minutes later absolutely all traces were gone. Due to its ease of removal, I am not entirely convinced it was actually rust.
While there is no excuse for a gouge in your original chamber, if your only concern with the new barrel is the brown staining, might be a really easy fix.
Good luck.
Kevin
AZ
I chucked a couple of sections of cleaning rod and a fresh 12 gauge brass bore brush in an electric drill, used a generous amount of WD-40, and 2or 3 minutes later absolutely all traces were gone. Due to its ease of removal, I am not entirely convinced it was actually rust.
While there is no excuse for a gouge in your original chamber, if your only concern with the new barrel is the brown staining, might be a really easy fix.
Good luck.
Kevin
AZ
We'll see.........