Well it seems that some people are going to shoot the longer shells in shorter chambers anyway.
On some of the drawings of the forcing cones, and the length given of the chambers by various manufacturers, I'm still going to abide by using 2 1/2" low pressure shells in 2 1/2" chambered guns because I still don't believe that the longer shell opens fully creating more pressure.

As to the statement made by vh20, I was the one that gave the statement of using a 3" shell in a 2 3/4" chambered gun as a comparison of 2 1/2" to 2 3/4". Nowhere did I state that it was low pressure, it was just an analogy of using a longer shell in a shorter chamber.
Also you stated that older guns were never in proof even when they left the manufacturer. Wrong, they tested their guns before leaving the factory with loads that were twice the powder and shot of the loads available at the time. (Hunter Arms Co.) They just never put any proof mark on the barrels until the 1920's on some.

"The reason we don't do this is the inherent PRESSURE of the shell, even when fired in a 3" chamber, not the length"

Actually the pressures in a 20 ga. 2 3/4" Heavy Field Load 1 1/8oz shot is equal to the pressures in a 3" of 1 1/8 oz of shot.(Lyman's 5th Edition) 2 3/4" shells also had Magnum written on them.
Yes the loaded shells do cost more to buy, but there are companies out there that sell componants including 2 1/2" shells for reloading.

It's like one previous poster said, why encourage someone that is new and has a 2 1/2" chambered gun to shoot longer shells in a shorter chamber, it must be ok becasue he read it here.

David