Thanks for posting that photo. I had the identicle problem with an old American gun, and posted about it, here, but, I had no photographic evidence.
The cure for my gun was a simple lengthening of the forcing cone, NOT the chambers. The cones in that gun were rather short and were more like "steps" for lack of a better word. I did have regular posters, here, reply that they doubted what I was describing could actually happen.
Ted, Charles Fergus reported that problem, also with a cone that was more like a step, in an article on Damascus guns he did for Shooting Sportsman. His problem came with the 67MM Brit shells in a 19th century gun. Same shells in a 2 1/2" Brit gun from the 1930's: no problem. In his case, he found that true 65MM shells (don't recall that he specified a particular brand) worked OK in his 19th century gun. I believe, but can't recall for sure, that one or two others here may have reported a similar problem with very short cones.