Re the shear stress on the cross pin. We are all assuming that the firing force is directed equally onto the breech face by the cartridge and onto the pin.
But, the pin is rarely unsupported in a double gun. A fair amount of metal is in FRONT of the pin, the knuckle, in full contact, thus any force exerted on the pin will be channeled to the action body. The same goes for OUs with bifurcated pins, they are supported at the front via protrusions of the action body metal. The pin undergoes deformation, more than shear stress. It is worth pondering whether this deformation comes from the forces of firing or via the recovery phase of the action body. If the action flexes back then it must spring back to its original form, squeezing the barrels between breech face and cross pin.
Personally I disagree with the assumption (fostered by Greener, Burrard and Thomas) that the cartridge head acts alone in pressing against the breech face. Whatever happens during firing is, I believe, far more complex than a simple thrust on the breech face by the cartridge head, and an equally direct and simple thrust on the cross pin. Over the years I have seen some strange damage to double guns but have yet to see a sheared cross pin, even in folding shotguns that have totally unsupported cross pins.
