Since the short mags came out I have been playing with necking them up or down. Necking the .325 WSM up to .458 has worked out very well. I have built three rifles chambered for this cartridge and with 20" of effective barrel length it equals the .458 Winchester mag ballistics. The 450 will easly push 300 grain Hornady H.P. at 2650 with 61 graing of IMR 4198 and the 350 grain round nose bullets at 2350 or more. Either of these loads could be hotter but looking at the velosity range these bullets were designed for I think I would be pushing bullet failure.
Come spring I am going to try the 480 and 500 grain bullets. One thing that works out great if this cartridge is chambered in a standard length action like a Mauser is that unlike the .458 Winchester you can still seat the bullets out far enough so as to not loose case capisity.
Another nice point is the loading dies. I made up a long expander to start the necking up process, this takes he brass to .450 diameter then using a long tappered expander gothe rest of the way. The seating die just had to be opened up a bit. With a bit of simple machine work that even a small shop like mine can do you elimate the need for custom dies.
Bye he way, as a friend/customer of ours found out this cartridge flattens both black bear and deer, sort of BANG FLOP!