Longer shells in short chambers won't raise pressure much. This practice can however add "felt" recoil. Case in point, I shoot 3" shells in a 2 3/4" Parker for ducks with no noticable effect in recoil. Alternatively, I have a Lefever 20 gauge that had 2 1/2" chambers. Boy would it kick with 2 3/4" shells!!! I had those chambers lengthened to 2 3/4". The problem may have been due to the angle of the forcing cones but I have no data on this...
Sherman Bell published his tests of long shells in short chambers in recent years in the Double Gun Journel. The maximum effect in pressure was for one gun/shell combination was to raise it by 1000 psi. Most combinations were much lower than that.
Mark
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