Zapata,
My 470 is a Belgium made SxS pre war.
My custom double is 400-350 NE O/U built on a Ruger Red Label Stainless 20 gauge action.
This action is often referred to as a "boxlock" by some who don't know as much as they think they do!

In reply to:

---

I envision either a .470, or the .45-120 (3" case, same powder capacity as the now-popular .458 Express, which is a 3" 458-style Magnum case, except with the same rim as the .45-70). Hulls for the .45-120 are readily available, and relatively speaking, dirt cheap. You should be able to crank out 2200-2300 fps with a 500 gr. Hornie Solid in this length case while keeping pressure 'way down there. Whacha think, guys? Everyone's opinion is welcome and valued.

---

The 450 Nitro Express pushed a 480 grainer at 2150 for a pressure of 17 tons.
17 tons is "up there" as far as double rifle pressures are concerned.
The 450 NE has a larger case capacity than the 45-120 so I would not be trying to achieve 2200-2300 fps with 500 gn bullet in the 45-120 case size in a double!
I would suggest useing 450 NE, 450 NE No2, 500-450 NE or 470 NE.