RigbyMauser is on the right track! If you must use a shotgun to build a double rifle, the the safest place to start is with a German S/S shotgun. The Germans have always built firearms stronger that was needed, in break top guns & rifles. This was because the multibarreld arms have always been their choice for hunting. This way the didn't have to build two different action, and could make a shotgun, or rifle on the same action. The American, and British tend to build a shotgun just strong enough to handle shotshells, and in fact British shotguns are built exceptionally light, and even high brass shot shells are hard on most of them. In Britton, shotguns, and double rifles are diffinately built on different actions.