My first suggestion is to buy a spare barrel-or 2. I have a Star Model B myself. You did not specify if this is an original or aftermarket barrel. Try another barrel,if that works fine you know the first barrel is at fault.
As far as cleaning up the chamber,I would try an abrasive or polishing compound-a very mild grade, and very gentle polishing by hand.I would put some compound on a cleaning ptach wrapped around a wooden dowel that fits the chamber very snugly, then rotate in one
direction only 3 times, then check for fit and function.