Mickey;
A very interesting set of questions and, for me,
very timely. I just bought a DR that was rechambered from
.425WR to a 450/400 case trimmed to 2 3/4" and shooting
the original 410gr. bullet. It is not re-proofed. I would
like to get it proofed in the future, but I don't think
the English proof houses will even do wildcats. I paid
about 25% of the price this gun would sell for if if was
still an original.

500grains; I think your post is right on the mark.

As for a DR on a shotgun action. No thanks. Proofed or not.
Raff