Tony,
Glad you responded to this one as I know you've been "dancing" with this cartridge for a while now.
Here's the basic plan.
I really would like to build a 5500-6000 ft lb 577, something like a "heavy nitro 650 gn bullet" load, for want of a better term.
Maybe 95-100 cordite equivelant but with the 650 gn bullet.
I realize that the pressure is going to be up there a bit and that was the reason for considering the 3" case.
With the 12 gauge frame and monobloc I'm thinking of using I will only be able to get about 235 to 240 thou max chamber wall thickness.
Not at all sure if that will be enough so will have to do the calculations.
I'm certain I can get the weight to at least 11 pounds and hopefully 11 1/2 and still maintain good balance etc.
I've had this idea in my head ever since reading your fat lady story and have been planning and figuring and measuring and drawing and calculating for some time.

It is still all just a dream at this stage.