Tom, a lot of research has gone into the question of shooting 2 3/4" shells in shorter chambers. Most of the studied results found that there is no appreciable increase in pressure when the longer hull is used. In fact, your Sterlingworth and Model 12 were both probably made after the 2 3/4" hull had become standard. Many gun manufacturers wanted the lip of the hull to open slightly in the forcing cone. The idea was that it would form a better gas seal. Do yourself a favor before letting a gunsmith do something irreversible: from the Double Gun Journal order Volume 12, issue 4. It has a well researched article by Sherman Bell titled "Long Shells in Short Chambers". I think after reading it you will conclude that the length of hull is not nearly as important as the pressure the load produces. You can call the DGJ to order at 1-800-447-1658. Good shooting!
