I think there is a horsefly in the yogurt on your explanation as well, Jim. If one has a short chamber gun, and uses 2 3/4" ammunition, only to discover the cases ripped off down to the contact length at the cones, velocity will not automatically increase-but, recoil will. I suspect pressure would, also, but, I can't prove it.
A friend owns a French Charlin that rips the cases down to a bit below 2 1/2", sometimes even with European spec 65mm ammunition. To say the gun is unpleasant to shoot is an understatement.

Best
Ted