One thing I would like to insert into this thread;
"Years" prior to Bell & also some time prior to Thomas as well, actually the late thirties, British ammo makers pretty well settled this matter. Upon introduction of the "Fold"/"Pie" crimp overall length of the "Loaded" cartridge became shorter. To accomadate the same load internal capacity had to be increased. Whether they acomplished this by reducing base wad height, wad thickness or wall thickness or a combination of these I have no idea. Anyway there was then a fear that these shorter shells (nominal 2½") would be mistaken for 2" shells due to their shorter length. Subsequently, experimentation was carried out with case length to give a similar "Loaded" length to the older roll crimp shell. Testing revealed that as long as the load was the same pressure & balistics remained virtually unchanged. "NO" drastic change was noted in pressure, velocity or recoil. All of this is reported on by Burrard. This work was not done by Burrard himself, but by the makers. Burrard was not so Egotistical as to think results could only be determined by "Himself", but relied heavily upon those in position to "Know". I do not have any old fired English cases to measure, but it is my understand that from approximately WWII time on all British "Cartridges" "Designed to be fired from a nominal 2½" chambered game-gun closed with a fold crimp have a "Fired Length" longer than the chamber they were "Designed" for. "IF" this can be proved not the case I will then stand corrected. As stated several times here the answer is "Designed Pressure", not case length up to a point. The British's conclusion was shells should "NOT" be used having a "Loaded" length long enough to enter the cone, as this could very well inhibit the opening of the crimp, leading to excessive pressure.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra