I would build a .458 Winchester. I can load mine down using.45-70 bullets, and it is fairly enjoyable to shoot. I particularly like the 405 grain Remington JSP bullets at about 1600 fps, which are available in bulk for a reasonable price. Or go all the way up to something designed for elephants and rhinos. I have even shot hard cast bullets at 1200 fps. Brass life has been very good with reduced loads.