CIP, the Proof people, have issued a series of guidelines on minimal wall thickness on various types, among them double rifles, and that might be useful for designing. I am told that this info is now available free of charge at the CIP website.

A thought re shotgun to DR projects- the strongest action is a back action hammer gun. In addition to a solid bar it offers the benefit of simplicity, there are no timing considerations, ie cocking, ejection, etc, to complicate matters for the first timer. If the action is well regulated and spring power is balanced the hammer gun can be surprisingly easy to manipulate. It also has the benefit of rebound action and incorporates a passive safety system that prevents hammer to firing pin contact unless the trigger is pressed.

Back action hammer doubles were produced in their thousands by various Italian makers in the 50s to 70s, some of these firms have become famous today, like Armitalia of the Lucchini family.