Mac,
Thanks for your post.
I realize my post goes against common beliefs regarding double rifles, however I [and Mac] assure you my info is true.
I must admit I "stumbled" upon it by accident. When I was test firing my 450 No2 at Champlin Arms I wanted to see if it would shoot 350 grain Hornadays as I knew I wanted to use the double on deer and pigs. I took with me to Enid IMR 3031 and 350 gr Hornadys. I started with the same powder charge that George told me was safe with 480/500grain bullets as I knew it would be a safe load with the 350's.
To my suprise the 350's shot right with the heavy bullets.
I later tried 300 grain bullets in my 450/400 3 1/4" with the same powder charge that regulated with 400 grain bullets. EUREKA!!, they grouped right with the 400 gr Woodleighs.
Then I began some research, and applied some mathamatics. What I rediscovered [this must have need discovered in the early 1900's] is that if you take a bullet of approx 75% the weight of the "regular" load and use the same powder charge it will hit to the sights and regulate as good as the load the double was made for.
Example: 450 No2 480grain bullet X 75% = 360 grains.
475 WR 520 grain X 75% = 390 grains. [The WR load was 385 gr.]
450/400, 400 grain X 75% = 300 grains.
470, 500gr X 75% = 375 gr.

This formula has worked in my doubles and in another 470 Chapuis [350 Hawk with 75 gr IMR 3031]. I will test it in another 470 Chapuis in the near future.

If for some strange reason your double will not shoot lighter bullets properly, all hope is not lost, Just get some Hawk bullets in the standard weight with the .025 jkt for deer and pigs.
The secret is to match the bullet to the game being hunted.

As another strange coincidence In my 450 and my 400 the lighter bullets both chrono @2330fps. If you check the ballistics of the lighter bullets with the heavier bullets they have the same trajectory to 300 yards. Thus the 350 Hornadays at 2330 fps have the same trajectory to 300 yards as the 480's at 2150fps. It must be MAGIC, or is it mearly physics? All I know is that it works.
If you will recall the 375H&H was known to shoot the 235, 270 and 300 grain bullets to the same point of aim and they all regulated to the double rifle. This is because they were loaded THAT WAY, instead of to the max velocity possible with the lighter bullets.

PS
Capt Curl...Who is the Nimrod???

Send me that 500 Nitro and I will work up you a good deer/pig/black bear/plinking load.
MMM... Lets see, a @ 425 grain bullet with @ 80 gr of IMR 3031. I will even work up a Nitro for Black load [very light recoil] for you.

It is all elementary Physics my dear Watson.
Or maybe Magic.