looper,
Well, for one thing, the Rigby will fit in the magazine better. Weatherby's belts are purely cosmetic. Since his cartridges were all blown out to a sharp angled shoulder for headspacing, there is no earthly use for the belt, except for marketing purposes. What it does do is cut your magazine capacity by a complete round. Weatherby .460's by Weatherby only hold 2 rounds in the magazine and once you've faced a really angry potroast, that is a serious deficiency indeed.

Now as to the forming question. I simply take the .416 case and run a .458 expander die through it, then fireform. It doesn't get much easier than that. If you really fall for all the hype of HV, you can load up the Rigby to whatever mind-blowing velocity you want but the game won't be able to tell the difference at 60 yards or under. And unless you are shooting monolithic solids, you'll get better bullet performance at 22-2300 fps than you will at 2500+.