Felix,
An ideal weight for a .500 nitro would be around 11+ pounds; anything lighter is too light for recoil concerned. However, overall weight isn't the whole story, but BALANCED weight is. By balanced weight, I mean your goal for a best balanced, nicely handling double rifle, is to have most of the overall weight between the hands, when in firing position. W. Ellis Brown, who wrote that book we previously mentioned, made his very first conversion using a German made 16 gauge action to .450 nitro. As soon as I picked it up, I could feel that the barrel weight was too extreme, too heavy toward the muzzles, too large in diameter toward the muzzles; resulting in being not balanced well at all. When I mentioned that to Brown, he said that he agreed, and that he would never again use a 16 gauge for that or similar caliber, as it was simply too light in weight, forcing him to make barrels excessively too heavy, in order to handle the recoil of that caliber, even though the action was able to hold up to the firing forces, pressures----he had fired it hundreds of times, if I recall correctly.