Well, the .416 Rigby a tapered case with a sloping shoulder? I have not yet seen a case with less taper and a sharper shoulder angle, I see very little possibility of "improving"it! BTW, the 9.5x57Mannlicher-Schoenauer aka .375NE shows even less taper than the 9.3x62.
As I told above, there is no need to open up any standart length Mauser 98 action or magazine to handle the 9.3x62! This cartridge was designed "taylor-made" to use the available magazine length of the standard military 8x57S actions to it's capacity. Of course, if you want to use longer than standrd bullets seated out you better use a true Magnum length action from the start! The opening-up of standard length actions for cartridges like the .375 H&H, even by factories like FN or Parker-Hale, is nothing else than a makeshift expedient to avoid the cost of making a true magnum length action.