Pugwash,

Thank you for your input.

It appears that a double rifle needs to be wider at the breechface and bar to accommodate greater barrel/chamber thickness (due to greater breech pressures than a shotgun), deeper in the bar (at the rear) to resist flexing in the elbow of the breechface and bar, longer in the water table/bar to add material between the lumps/bites. Larger in the hinge pin.

A dolls head extension/greener crossbolt would stop the barrels trying to lift and push forward while the elbow of the breechface/bar was "flexing".

You don't like hidden third bites. I'd say, along with side clips, the hidden third bite stops axial movement...….but the bulk of the bulk of the movement/stresses would be longitudinal - along the bar (?). I'm aware that I'm getting into territory here that is out of my "engineering" depth.

Back action sidelocks, as against bar action sidelocks, are preferable to leave more strength/thickness in the bar/elbow area of the action.

This is just my summary and I don't presume to speak for you nor "lead the witness".

Re the double rife built on a shotgun action debate. Bearing in mind the above factors one still has to wonder how heavy does the action have to be in these areas to still result in a reliable durable double rifle. Remembering that it would be rare for a carry around hunting double rifle to be a rare article to have been fired 1,000 times. All being equal it would be the pressures involved with a double rifle cartridge that would be the determining factor.

I find this topic fascinating, and again, I thank you for sharing your knowledge Pugwash.