First off, to do a rimless version, it has to headspace off the case mouth
and that does not allow for higher pressure. Sammi says to keep the 450
Bushmaster down to 35000 cup and the 444 to 44000 cup......If you have
some machine skills, doing a bolt action 444 is not that bad. A .473 bolt
face would need to be re-cut. The tricky part would be to cut a slot in the
breech face to facilitate the extractor .

I do believe that a 450 Bush Master is chambered by Savage in a bolt gun.
The 50 Beowulf would be something to look at also.

350 Legend could be considered as a turn key option. It's capable of higher
pressure then most other straight wall options.