My question is why bother with rifling at all?

I can shoot 75 yards easily with my smoothbore 12ga so getting the same or better accuracy from a regulated 20ga is very possible.
It's also a more flexible gun like that, maybe look at putting a 1/4 rib with open sights on a 20ga and regulating it with screw in rifled chokes.

Loading brass cases with a load of the dirty black stuff behind a Svarog paradox slug or Lyman style slug will be about perfect for what it seems your after.

But the other side of the coin is there's, no better reason for buying a gun other then because it's what you want.

But from my own experiences I prefer my 12ga ball and shot gun over my double rifle as it's just such a flexible hunting gun that I can use on anything I'm ever going to hunt if I can do my part.