: Swaging .338's down to .330" is pretty straight forward. Corbin's sell such reducing dies quite reasonably.
: Swaging down is much better than bumping as the only thing close in an undersize bullet is .323" - too much of a bump in my opinion and full bullet making dies would probably be necessary - on top of which, bullet weights are limited to the lighter weights, mostly.
: Seems to my memory, the .318 Rigby is very similar to the .338/06IMP in size and capacity BTW.