You mention there remain rough areas in chamber after polishing. With old guns poorly stored, this can be common. These local rust spots can be quite hard. I have taken to giving those hard rust spots a good working over with fine emery paper on a dowel rod, sometimes even with a rounded tipped screwdriver, till they are gone, then polish again.
Often these areas of hard rust are in the last cm or so of the chamber, adjacent to forcing cone. They are there because this area gets coated with powder fouling residue on nearly every shot, and, often is not cleaned throughly, so, rust developes and gets compacted by subsequent shots.
Check Kent Gamebore BP shotshells. Think I remember some of them being the old standard 12 gauge load of 1,0 oz of shot and 2,5 drams powder. I have long loaded this 1,0 oz (28 grams) 2,5 Drams load with both fiber wads and one-peice plastic wads. It is an excellent load for shooting clays (including 16-18 yard trap) and flushed or descoyed birds. A 1 1/8 oz (32 grams) load is more for passing (flighted) waterfowl and not really needed for much else.
Niklas
