I think it stands to reason that, if a 375 H&H mag and a 450/.400 both generate 4000 ftlbs of energy then a weight of about 9 1/2 lbs should fit both !!?? This should relate to both double rifles and bolt guns..........afterall physics are physics !!!! A 9 1/2 lb .375 H&H is quite shootable therefore a 450/400 of the same weight should be equally so.

I would not expect much difference in weights between calibres generating similar ftlbs of energy e.g. .450/500, .450 NE, .465, .470........or 480gns @ 2,150 fps (?)

I'm sure the British gunmakers would have had a formula for the weight of each grouping of calibres/energies to recommend to a client..........but would defer to his instructions of course. I'm sure that somebody who was going to carry a double for the entire dry season and shoot 50 rounds out of it per year would want a "lighter" gun than somebody who shot 50 rounds out of it, in a lifetime, mostly target shooting from a bench/rests.

I agree with the comments about balance. Correct balance makes a heavy gun seem lighter and poor balance makes even a light gun a pig to carry.