I'd get a .750" fine grit (180) polishing hone, on a twisted wire stem from Napa- and using Kroil on it, and with the barrels locked in a padded vise, slowly work this in and out of the chambers, at a slow rpm with a variable speed electric drill- I did this with a 12 gauge L.C. Smith that had the same issue- what was the cause? Previous owner was too cheap to use snap caps, always stored his shotguns with the hammers down, and used old cut off paper shells in lieu of plastic snap caps- paper is hydroscopic, draws moisture-- hence rusty chambers- RWTF

"The field is the touchstone of the man"..