450 Rigby.

Brass and bullet availability. Also, I know from owing two 460 Wbys in the past that the 450 Rigby case will be able to deliver top accuracy with a variety of reduced loads that do not require fillers. In fact it would be better than the 460 with reduced loads because it does not have the huge freeebore of the 460

However, if the rifle was a Purdey or H&H etc then I would take 505 Gibbs.

404 for me would be out because of bullets and I think the 404 needs to be in the right rifle to make it all worthwhile.

Lastly, the 450 Rigby is not a classic cartridge and a jazzed up CZ is not a classic rifle and that coupled with all the bullets in 45 and being able to use 416 Rigby brass would make the 450 Rigby an easy choice for me.

Mike