Case volume in a shotshell is very different than case volume in a centerfire rifle case.
In a shot shell peak pressure happens before the wad leaves the hull, so you aren't using the entire hull volume.
Further and Tom Armbrust will confirm this....fire a 2 3/4" cartridge in a test barrel with a 2 3/4" chamber and fire that same 2 3/4" cartridge in 3" or even a 3 1/2" chamber and you still get the same pressure as you did in the 2 3/4" chamber test barrel. I know this from first hand experience. That is telling you that peak pressure happens very early on in the event and that anything that the load encounters past the hull does not affect peak pressure.
Another perfect example is firing a 3" cartridge in a 2 3/4'" chambered test barrel.......maybe a 500 PSI increase.
So going back to the original question.......and maybe why Tom Armbrust answered the way he did was...... IF you stuffed a load with the proper wad column height fit for a 2 3/4" fold crimped load into a 2 1/2" hull with a fold crimp, YES you will increase the pressure. Why" because you just collapsed the wad and upon ignition you will not have the same amount of space for the wad to collapse into thus recording a higher peak chamber presssure. Depending on the wad and payload, how much the wad collapses before the crimp yields influences peak chamber pressure.
Those are some of the reasons why this whole subject is iffy, because too many things can affect the outcome. There is no one answer for all situations.
Look at some of the Euro loading data, since all they use are straight wall hulls the hull isn't even a consideration in their data.
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