Burch
There's an article in a past Double Gun Journal about shooting 2.75" shells in 2.5" chambered guns. According to the article, there may be very small pressure increase, other then that about all it will do is tear up the end of the shell. If your looking for low pressure loads I'd try Winchester AA Low Recoil/Noise shells or Xtra-lites.
Another thing, it's been my understanding that the British proof house does not consider a gun out-of-proof if you simply extend the forcing cones. Based on that, instead of extending the chambers on several of my 2.5" chambered guns I've just extended the forcing cones (with a FC reamer purchased from Brownells). I still use low pressure shells, only now it doesn't tear up the when I shoot the gun.
Steve
Last edited by Rockdoc; 01/20/08 10:37 AM.