Bugger,
As I'm sure you know, the .416 Rigby's one of the old British cartridges designed with excess case capacity so lower-pressure loads could be used, due to early Cordite being so heat-sensitive. The oversized case allowed plenty of velocity to be reached without the danger of cases sticking in the chamber in tropical climates.
Consequently original ballistics can be achieved at very low pressures with modern powders. The SAAMI standards are the original 2370 fps for 400-410 grain bullets at 52,000 PSI, exactly the same pressure as the .444 Marlin. So there's room for safe experimentation at under the 62-65,000 PSI of modern .416's like the Ruger and Remington.
