Not recommending it on an expensive gun, but I have solved this problem on several cheap Belgian guns by chucking a new square edged cylindrical fine grit stone in a Dremel tool and running it at a slow speed around the rim recess with the shaft parallel to the barrel. It often solves the problem without leaving any marks on the recess except for a shiny path. Often the needed treatment is extremely minor.

I would speak with Dean Harris at SKEETS.

Thank you everyone. I really appreciate your advice and comments.