Friend of mine had Lee make up up a set of dies for re-sizing .338" bullets down to .330 for his 8x56R Styer.

These bullets and die sizing system (3 dies) would also work well for the .318 Rigby.

One need only contact Lee by phone or through their custom shop for diameters and dies. They've been doing this for over 3 years now and should have a good handle on what is necessary.