I just found this on AR http://members.fortunecity.com/howda it shows a sauer shotgun converted to a Howdah in .45/70. This is similar to what I inted to do but with a Merkel shotgun in .500S&W (435g at 1700fps for above 2700me) I would go for the classic look not the fiberglass as shown . Check out the lump on this blokes forehead he got when both barrels went off together during R&D testing!
RLI