Ballistically, you are absolutely correct. However, that line of reasoning disregards the Quainte Qlde Brytyshe Customn of propriatory cartridges. Jeffrey couldn't use the .425 because it belonged to Westley Richards. All the same for Rigby. Besides, Rigby had the sole rights to Magnum Length Mausers so it was just good marketing to go with a cartridge that required such things.

Anyway, I hope the inventor has great fun developing loads and has great success on game with his new toy, however redundant the reactionary pukka-sahibs among us may find it.