NE450No2:
I am not at all uncomfortable with the notion that some (perhaps most?) double rifles can be made to regulate with a number of different bullet-weights and loads. My question would be...
Why?

Are you seriously telling us that a .400 NE, for example, will kill pigs, goats and deer BETTER with a 300-gr bullet than a 400-gr bullet? ..when the rifle is regulated and sighted for the 400-grainer? ..and the trajectory of even a round-nose 400-grainer will deviate no more than ONE INCH from the line of sight, out past 125 yards or so?

What happens when your bopping along through the bush with your .400 double happily stuffed with 300-gr loads, and the buffalo of your dreams steps out at 20 yards!?

The only way to 'use your double to best advantage' imho is to settle on that one good load, then practice Practice, PRACTICE until you can hit the vitals of any target animal from any angle at any practical hunting distance.

Light-bullet loads are for that class of reloader described in Mickey's post above IMHO. No offence intended, burning nitro for ANY reason needs no justification, its all fun after all!