Before Sherman Bell; Before Gough Thomas; In the year 1938 the fold crimp shell was introduced to the British trade. Wadding had to be adjusted to get the same load in the "Nominal" 2½" shell (actually about 2 9/16"-2 5/8" long). The length of the loaded shell was much shortrned & there was Legitimate concern they would be mistaken for 2" shells which actually went to about 2 1/8", but of course loaded much lighter. Very shortly experimenting began on putting the same load with appropriate wadding in a nominal 2 3/4" case, which when closed with the fold crimp gave a "Loaded" shell length virtually identical to the roll crimped 2½" case. It was found that both pressure & velocity were for all practical purposes "Identical" between the "3" methods of loading (2½" RC, 2½" FC & 2 3/4" FC) all loaded with the same load. WWII prevented extensive testing at the time, but was continued immediately following. To the best of my knowledge from that point forward "ALL" British shells of a "Nominal 2½" length, closed with a fold crimp "Have been Longer than their Chambers". If you disagree give me proof.
All this was reported on by Burrard. Burrard did mention testing some few shells himself, but essentially reported the work of the British Shotshell industry. Previous to this he had warned against using 2 3/4" shells in 2½" chambers. This was two fold; First the longer "Roll Crimped" shell would actually enter the cone "Before Firing". There is a strong possibility of this delaying the opening of the crimp thus increasing chamber pressure. Second these shells carried a heavier, higher pressure load than the 2½" chambered gun was designed for.
Unfortunately both Thomas & Bell in their reporting honed in on statements Burrard made on using these heavier loads & totally ignored the actual facts of using appropriate loads in the longer "Fold Crimped" case. Bell was not even sure that Thomas' tests were conclusive & needed him to "Prove" them.
In point fact the matter was settled in 1938 & the "Billions" of British long hulled, fold crimped shells marked "For use in 2½" chambered guns" (paraphrased) should be sample enough.
To the best of my knowledge none of the current testers have addressed the question of using a shell which is actually long enough to enter the cone prior to being fired. Until such testing is done "Personally" I would strongly advise against doing so. One powder makers handbook stated that pressures could be varied very widely by simply changing crimp depth, thus strength. The hull being squeezed by the cone prior to firing has the effect of increasing crimp strength.
The testing which has showed no pressure concerns has all been done with shells having clearence prior to firing & the hull only laps into the cone after opening in a normal fashion. By this time the shot is already beginning to move, but without the space & speed to create an obstruction condition.
