To the best of my understanding from the beginning of the fold/star crimp in the late 30's british shells sold 7 marked for use in "2½" chambered guns were all longer than the chambers. This was done for a two-fold reason. The new crimp resulted in a shorter overall length of the loaded shell which reduced the internal volume resulting in a shorter less effficient wad column unless the "Fired" length of the hull was lengthened giving a similar "Loaded" length to the older roll crimped load. Also it was feared the shorter loaded length of the new crimp would make the shells appear to be of a 2" length & be used in these lighter wt guns with possible disastous results.
The OD of a shotgun bbl normally tapers at a more rapid rate than the chamber body itself, but not as rapid as the forcing cone. There are few bbls existing in which a thinner wall would be produced than that of the end of the chamber by lengthening the cone unless it were carried to some extreme length. Moving the chamber end forward will result in a thinner wall. This is why some proof laws require re-proof for a lengthened chamber, but not for a lengthened cone.
Just personally I would not fire even that first shell loaded with a slug or other solid projectile from a bbl in which I knew the hull length was longer than the chamber. This presents an altogether different circumstance than firing a load of shot which can flow through the slight reduction of diameter. I would likewise not fire a shell which had a loaded length long enough to reach into & engage the cone prior to firing.
