The theory is that surfaces other than the hook joint absorb the forces and thus relieve the hook. Such a surface is the front side of the rear lump which is "fitted on the circle" and abuts against the action bridge, thus receiving the forces that arise on firing.
Two points there. When a larger hinge pin is fitted due to wear the barrels are moved back enough to eliminate any contact between rear lump and action bridge. There is no longer any "fitting on the circle" yest such repairs last a long time without developing looseness.
Second point is that folding type shotguns, as well as some less expensive non folders, such as the Midland Gamekeeper, have no central action bridge. In other words the cross pin bears the total force of firing, yet such designs seem to last a long time without loosening.
I suspect that the pheonomena that develop during firing are more complex than commonly protrayed. The dynamics involve more than simple rear thrust of the barrels and flexing of the action bar. There is, among others, the Poisson effect of axial contraction and radial expansion of the chamber ends of barrels (thick wall cylinders) during firing.
As to to the original post, the solid cross pin has not proven more durable than bifurcated pins in guns of equal quality, compare Superposed with Perazzi for a comparison.
The other intriguing fact is how autos and pumps never develop action looseness. Makes you wonder why.
