For anyone thinking about building a double on a shotgun action here are a few things to think about.

For a cartridge like the 375 flanged magnum, chamber pressures could be in the region of 50,000 psi or higher.
If a proof load is used at plus 30 percent this will mean that the chamber pressure could be around 70,000 psi for the proving load.
That's seventy thousand pounds per square inch.

I think it unwise and unsafe to calculate any monoblok to barrel join shear rating at 100 percent.
To do so assumes that the solder join is 100 percent perfect and as the join is hidden from view an assumption is all you have, there is no visual proof.

Shear strength should be calculated, but when dealing with a cartridge being fired in a barrel chamber there are other important stresses on the barrel to monoblok join which need to be considered.

Back thrust / axial force is directly related to pressure and is directly proportional to it.

While axial force needs to be factored in to the safety calculations, so does chamber pressure as both can affect the ability of the soldered joint to withstand the loads and stresses it must endure.

Chamber pressure alone (in home built doubles) can in some situations degrade the solder join when a soft solder is used between barrel and monobloc.

The bottom line is that you may never get any warning that the join is weak or has weakened.
IMO this is a risk far to high to accept.

Are there are any manufacturers who build double rifles on the monobloc system using shotgun actions and who rely on nothing more than tin/lead solder to hold the barrels into the monobloc?

More later........