Thanks for your input. When I mentioned the switch barrel I was refering to using the standard shotgun barrel and the rifled barrel not changing calibre. I may have got a lead on a GP now but I'll see how it pans out. The main difference I can see between a MH/ME and a GP is the recontouring of the breech block to allow a 12G round to sit flatter in the action with the block down so the round will feed easier as can be seen in Marrakai's photos. I'll have a play around and see if there are any other reasons that it may not be adaptable.

thanks,