See this pipe gun:

http://youtu.be/7Va87gB_4AI

Obviously there is NO hinge pin on this construction. Observe how both the barrel and the outer "breech" pipe recoil together, dragging the "breech" backwards.

I suspect that the expanding shell sticks to the "chamber" and barrel and shell casing act as one unit pushing back on to the "breech". If something similar happens in double guns then the strain on the cross pin is minimal. From this point of view the friction coefficient of the chamber would become a factor to consider.

This is a fascinating subject and really could benefit from some high speed video so we can see what really happens to the breech during firing.

In my previous posting I mentioned the Poisson effect. Thick walled cylinders expand radially under pressure and contract axially. Obviously after the pressure is reduced the cylinder returns to its original dimensions and this recovery must involve some force on the breech face and cross pin. As far as I know there has been no analysis of this phenomenon. Most authorities focus on the breech flexing, assuming that there is no dynamic processes going on in the barrels. The authorities consider that the pressure on the breech face is via the shell alone, while the pipe gun tends to indicate that barrel and shell together press the breech face. Just looking at the imprints left on the breech face of double guns seems to indicate that there is some pressure exerted by the barrels too.