Pod this is not quite my field but I have seen several solutions to your problem and I'm sure there are guys in the US who can put it right for you permenantly and safely.
Jim Legg say's only an idiot slams a gun shut.That may be true but there a lot of idiots about.
My comments are based on 35 years of jointing and re-jointing of double guns ,{ 4 so far this month and 3 to do next week}so I see all sorts of problems from all sorts of causes.Including previous work done by amature/DIY "gunsmiths"
There is no single reason .It happens .