C's commissioned rifle will be a Boss under and over double rifle. Prior to C's U/O rifles,he mentioned to me Boss had made only the mention .375 back in X.

IF it was mine, and a Euro calibre, the following COULD be choices:

the obvious and very useful 9.3x74R;
that cartridge necked down to 8x75R;
something I wouldn't choose, a 10.75x68 Rimmed - does it exist?
the modern .500/416 NE;
many medium rifle choices.
I'm sure there are other choices?

Imperial choices:
a .450/400;
something I would choose, not having one, a .375 Flanged;
matching the .500/416, a .416 No.2 Rigby;
I would be attracted to the .333 Flanged Jeffery.
Lots of larger medium bore rimmed choices.

C feels an U/O is less suited to a very large bore, and is more Continental European. Maybe the Ferlach gunmakers might disagree on the first?

Another complication for C is he likes to have the BOSS serial number to match the calibre e.g. 10375, 10600 etc. The prefix 11### might be an option. A shotgun and rifle can use the same Boss serial number. One of C's rifles wears the same SN as one of Eric Clapton's shotguns.

This rifle would be the final order of C's 15 year, so far, Boss project.

Keep suggestions flowing.

I like the .333 Flanged. A 300 gr .333 at 2200 fps is still very effective. I would get an assurance from Geoff McDonald that he will resume, one day, making projectiles for it before commissioning a new rifle though first. Useful without projectiles for hunting. Bertram makes useful .333 projectiles for ferals and targets as well.