GunC: The Ruger #1 Tropical chambered for the .416 Rigby is a great shooter. I had one follow me home years ago - had no real use for it but wanted an "African" gun just to have.
Pressures - yes, factory loads tend to be in that 35-40K psi range. The maximum allowable pressure is higher at something near 48K - I get that info from reading The A-Square reloading manual.
The people doing their load development used three powders: IMR 4831, Hodgdon 4831 and RL- 22.
Both the IMR and the RL powders produced pressures in excess of 48K psi with velocities at 2481 and 2467 respectively. (400 grain bullet). As I said before, I wonder what pressures would be at 2600 fps. The Ruger action is very strong but that is a very large case with a lot of bearing surface.
Pete

PS - just ran some numbers. 400 grain at 2600 fps = 6000 ft.lbs ME