Personally, I have never built a .500 on any Spanish action, though I have built a few of them, and many more .450 nitro, on German frames, and on some larger waterfowl English action frames. ONE OF THE MAIN PROBLEMS WITH USING A 12 GAUGE FOR A .500 IS GETTING THE COMPLETED OVERALL WEIGHT TO BE CORRECT, HEAVY ENOUGH, FOR A .500. USUALLY, THAT MEANS THAT, TO GET THE REQUIRED NEEDED WEIGHT, ONE HAS TO HAVE OUTSIDE BARREL DIAMETER CONTOURS LARGER THAN PREFERABLE, NORMAL, MAKING FOR TOO MUCH FORWARD BARREL WEIGHT--A BARREL HEAVY DOUBLE, NOT THE MOST PREFERABLE RIFLE DUE TO WEIGHT ISSUE. A 10 GAUGE MAGNUM IS PERFECT, FOR WEIGHT, ETC., IN .577 CONVERSION, WHICH I HAVE MADE SEVERAL, USING SPANISH BOXLOCK ACTIONS, BUT THAT SAME 10 GAUGE, OVERALL, IS TOO HEAVY FOR .500, WHILE THE 12 GA. IS TOO LIGHT FOR .500; THAT IS THE UNRESOLVED ISSUE, IN TRYING TO MAKE .500 CONVERSION. I have a beautiful 20 ga. mag. sidelock Spanish double, with 9.3x74R barrel set made by Francotte, so even "big time" makers did such conversions.