Oh, you mean like this? You're a hundred years too late!

As the guys have said, its all about back-thrust, not just pressure per se. Building a .600 full nitro double on a SxS shotgun action would run a high risk of either stretching the action or cracking the action-bar at the radius.

...and getting the weight up to the required minimum would result in a ponderously barrel-heavy pig of a rifle IMHO.