I had my .458 re chambered to the .460 G&A #2 which is almost the same as the .500/.450 Magnum works great and is very accurate but the recoil with the 500 grain bullets is a bit much from a rest. I don't have the figures handy as they were lost in an old computer but the velocity was half way between the .460 Weatherby and the .458 Winchester at about 2150fps.
The load I use is 87 grains of IMR 4064.
Cheers,
Laurie