I have been through exactly the same with my W J Jeffery sxs 410. Have had the action tightened but the bores are heavily pitted. Phil Smith at Bozards who did the action work was reluctant to clean up the barrels as in his view would be taking away metal and would need to take away a lot to get really clean barrels. His advice was just to keep on using it and one day either have new barrels or have the existing ones sleeved.

It seems to shoot OK and through decent patterns. But cleaning regime does have to be more through, and do store it with barrels well oil and muzzle down.