Several people have also used the Model 21 for such a conversion. You won't find a stronger receiver. In fact Winchester had a Model 21 experimental double rifle and made a few. Andy's custom shop has also recently made such a conversion a few years back and it was featured in a DGJ article if I remember correctly. From what I remember they take a 20 gauge and 1894 (?) octagonal barrels and round them off to fit. I believe Russ Ruppel even made one several years back. But you have to remember that the cost of a double rifle might not be that much more than to have one converted. Maybe a used one. (?)