guyandarifle,
The Rigby case actually doesn't have quite the capacity of the .416 Weatherby, despite the Rigby's lack of a belt, due to some other minor changes in shape.
I did an article a few years ago for HANDLOADER that partially involved loading the .416 Rigby to "modern" pressures, and after some calculating decided 50 fps less than .416 Wby. velocities should be doable, which amounted to 2650 with 400's. It worked fine in my rifle, a CZ 550 Magnum, and recoil really wasn't all that bad. But I've never hunted with the higher-velocity loads, and in fact when I went Tanzania a year or two later used the standard-velocity loads. They killed a Cape buffalo nicely, just like they've been doing for over century.
So it would appear that brass or freebore really aren't a big issue. (with 50fps-ish being the difference while still safe)
Appreciate the input. I'm not really trying to make a Rigby into a 'Bee but it's nice to think one should safely be able to explore up into that general area without profound concerns you're pushing the Rigby that hard, assuming usual precautions of course. (mine is also a CZ 550)
