felix,
What overall rifle weight, in pounds/ounces with no added lead or mercury weight in stock or elsewhere, did you end up with? When using ANY 12 gauge action for .500 NE conversion, I think it always best to NOT use the existing shotgun barrels for monoblock, but to make, from scratch (as explained in Ellis Brown's book) a new NON TAPERED monoblock for .500, enabling preferable outside barrel diameters, for needed balanced weight, as was reviewed in conversations above.