JPK,
I was only replying to your comment;
"The only NA critters I can figure where a 577 BPE or 12ga round ball rifle has an advantage over a 500 BPE or 20ga shooting either sabots or Fosberry types is brown bear and Yukon moose. And then, there are much better coices.
JPK"

I don't know of many better choices than the .577 BPE for big moose or bears.
I fully agree that the 20ga. double would be a great deal of fun to shoot and load for. And living in the lower 48 will be more than enough gun for anything in close. The new 20ga. double doesn't have nearly the alure to me of a classic double rifle but it's not nearly the cost either. There is no right or wrong, just a matter of taste and it's your choice.