"Well, the .416 Rigby a tapered case with a sloping shoulder."

Sorry about that - I meant it had a grossly tapered body, not the shoulder. It has a quite sharp shoulder, but way more taper in the body than needed. It is an antique design meant to provide extraction when unstable, temperature sensitive powders were used, ie: cordite. Since we've become familiar with improved design cases, we've learned that the straighter the case, the easier the extraction as bolt thrust is reduced greatly when high pressure is encountered. My new .375/06IMP, with 78 to 81gr. capacity depending on case make, exceeds 1912 .375 H&H ballistics by 70fps with 300gr. yet the cases literally fall from the chamber with "0" pressure signs. It has a mere .005" per side, taper.

The 9.3x62 has .019" total taper, or .0095" per side. This was measured on 2 fired cases from a Sauer rifle fired with factory Sako ammo. My own 9.3x62 Mauser, made in the 1920's, shows .472" base and .454" shoulder, for .009" taper per side. I'd say that is about even with the 9.5x57.

The .416 Rigby has .050" total taper, or .025" per side.

Yes, I am aware the 9.5x57/56 MS (as per COTW) has a mere .0095" taper per side - which is a very good design.

As to 'improving' the Ribgy, removing that gross .025" taper is what 'improving' the case shape is all about. Except for the taper, is very close to a beltless .416 WTBY(or visavis), only with a lot more taper. It would be a simple matter to improve that taper.

Now, whether incresing it's capacity and therefore velocity will improve it's killing power, is merely an argument over whether the .416 WTBY possess better killing power than the Rigby as it is now. With less taper, the Rigby will duplicate the WTBY.

The WTBY possesss the same taper as the 9.3x57MS & 9.3x62 - only .010" per side, btw.