Stan's comments reminded me of a Marlin 9mm Camp Carbine I once had. While the 9mm from a pistol is a real pussycat, when fired from that carbine it had some kick. That may well be the case with the .500 in a handy as well. I'd go the recoil pad route first, Limbsaver or better yet a X-coil pad from Hi-Viz. If that don't work cut yourself a 8" long piece of 3/4" copper pipe and fill it with lead. It fits into the stock like it was always meant to be there.
Don