Steve,
How do you go about shoe horning a .416 Rigby cartridge into a standard action? Putting a .404 Jeffrey in one seems pretty straightforward, but I just don't see how you can do it with a Rigby without compromising the strength of the action. Mauser used a 97.5mm magazine box with the .416 according to Jon Speed's book, and that's evidently the longest magazine they made.