I was thinking .416 Rigby too. When a friend got one from his wife for Christmas and I looked into it, having always fancied a .375 H&H (something to consider), I was quite suprised at how (maybe as or more) versatile the Rigby is!
Barret came out with their .416 to bypass the .50 restriction in California I'd read, but also that the US asked for something like it... Might be what you need indeed.
But wouldn't you prefer a nice retina-detaching .338 Lapua Magnum out of one of those fancy Savages? May NOT be leagl Downunder as the metric-named caliber is in use as a British if not Commonwealth sniper round.
Let us know how this develops -- this is cool!