Also remember the average 2˝" 20 gauge chamber measures 2˝" to the cone. Most 12 gauge so called "2˝"" chambers actually measure either 2 9/16" or 2 5/8" thus have a bit more clearance from the end of the loaded shell to the cone than does a 20. This clearance beyond the shell is what allows the crimp to unfold without a pressure rise. Since that "Additional 1,000 psi" with the longer hull was not consistent, but occasional, I have grave doubts that Bell ran sufficient tests to establish an absolute that it was the result of the shell length vs chamber & not just a normal variation from shell to shell. Within a given number of shells that much variation is certainly not unusual.
It has been "Said Many Times, Many Places" but I will say it once more "Never, Ever" fire a shell in which the "Loaded" shell end is pushed into the cone. This puts extra resistance on the crimp's unfolding & can create a rapid pressure rise.
If that shell end is pushed into the cone, you'll end up blowing the end off the shell, and you'll likely know right away that something is wrong. Most likely to happen in old guns with very short and sharply tapered forcing cones. I've fired a bunch of low pressure 20's in 2 3/4" hulls in guns with short chambers--including one Brit 20 with Damascus barrels (but a modern nitro reproof--3 tons--even though it was still 2 1/2") without having that happen.
I used to have quite a collection of fired hulls: true 2 1/2", 65MM, 67-67.5MM, 2 3/4". The interesting thing is that I never found a fired 2 3/4" hull that measured 2 3/4". All at least a little short. I reload a lot of Remington Gun Clubs, both 12 and 20. Just compared a once fired example of each. The 12 is just slightly longer, which would eliminate some of the advantage it would have in its more or less old standard American 2 5/8" chamber vs the 20's 2 1/2". But then if you're talking older Brit or European guns, they pretty much stuck with 2 1/2" for everything, regardless of gauge.
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