Hi Saddlesore,
I often lengthen the forcing cones in short chambered (2.5") guns so that I can shoot 2.75" shells without damaging the shells (yes, I realize that I still have to be careful to use low pressure ammo). It seems to work just fine and according to the British proof house, does not take the gun out of proof like extending the chamber would. I'm not British and that's never been an issue, however it is comforting to know that according to one nations experts it doesn't effect the safety of the gun.
Steve

Approach life like you do a yellow light - RUN IT! (Gail T.)