The lowest pressure Nitro Express round is the 450 No2.
With the standard load of 80 grains of cordite it was the lowest. The Tropical Load of 75 grains of cordite was even lower.
My 450 No2 is regulated for the 75 grain load. I use 88 grains of RL 15 with 480/500 grain bullets in my double.

Hits elephant, buff, and everything else, like the Hammer of Thor.