I assume that the Krieghoff S/S DR would function as well with .458's as my O/U has, but that still leaves me with reservations.

1. Anecdotal evidence which makes sense to me is that .458 ammunition which has been stored for some time behaves erratically, resulting sometimes in unacceptably low velocity. The .458 case is notoriously small for the performance claimed by the manufacturer, and duplicating factory velocities with handloads has always been difficult. Standard loads have to be heavily compressed, and the effect of compressed charges on ammunition stored for some time has yet, to my knowledge, to be investigated. It is entirely possible that reducing the air space causes ignition to be inhibited, resulting in drastically reduced velocities, certainly not desirable when dealing with dangerous game.

2. The other effect of the inadequate case volume is increased pressure, which can rather easily be dealt with by any modern bolt action, as well as some less than modern (my first .458 was built on a 1903 Springfield action). A double rifle is another matter. Every shot with a cartridge generating up to 55,000 pounds per square inch pressure puts a strain on the action, and with a S/S this strain is partially expressed as torque, which tends to twist the action lugs while at the same time placing a strain on the lugs, the action bolt and the hinge pin. Over time, this will put the action "off face". Using a cartridge designed for use in a DR will postpone this difficulty considerably.

Although I am not convinced that the Krieghoff .500/.416 3 1/4" cartridge is a totally sensible round for a DR, to me it makes much more sense than the .458. My choice would be the .500 NE, which can easily be down loaded to match .458 performance for practice and loaded up to use on game. There is a good variety of cast bullets which shoot really well in the .500, and you can be relatively assured that considerable practice will put far less strain on the action. Moreover, you can be sure that a factory load or round handloaded to factory specs will perform as expected, which is not necessarily the case with the .458.