I mentioned this in an earlier post and thought I might add a little to it.

When a double rifle is built from scratch it is possible to ensure that the thickness of the barrel chamber wall is adequate to handle the required plus 30% proof load of the cartridge to be used.
This will of course mean that the chamber wall thickness will quite easily handle the normal pressure of that cartridge.
On the other hand, when building a double rifle from a shotgun action where either the barrels will be sleeved into a monoblock or a cut off chamber section of barrels, the chamber wall thickness of the barrels alone may by necessity be quite thin.
Maybe too thin to totally contain the chamber pressure without some expansion taking place.
Sometimes this barrel chamber wall may only be around 1/8" or 125 thou.
This can be compensated or assisted by the fact that the monoblock will be surrounding and supporting the barrel chamber and therefore increasing the overall thickness of the total chamber wall.
Well maybe, maybe not.
If a clearance was used between the barrel and monoblock to assist solder flow,(a common practice) this would mean that a "sandwich" has been created where a thin layer (a few thou) of solder is filling the gap between the barrel steel and the monoblock steel.
If a hard solder is used in this sandwich there is a good chance that the total combined thickness of barrel chamber wall plus monoblock wall will be available to contain the pressure.
If a tin/lead solder is used in the sandwich and the barrel chamber wall alone cannot fully contain the pressure of a possible 70,000 psi proof load, the solder could be subjected to a compression load far beyond what it can safely endure.
If this happens the solder join will/could be severly compromised.

All just opinion of course.......nothing more