Another consideration with hot blued soft soldered barrels, in addition to the salt bath eating the solder away, is that it is pretty hard to keep all of the salt solution out of the void between the barrels, and if and when some of the salt residue gets in, those of us in the northern climates know what the salt on the roads can do to steel/iron. Taking apart a barrel set for relay that has been hot blued shows what the salts due to the barrels inside the ribs over time. Originally barrels were fluxed with rosin as the flux and the rosin is an inert material, and providing no water or bluing salts have gotten in, the soldered are of the barrels look the same as when they were soldered together.
Sometimes a ribs can pop loose, forend loop loosen, and if the barrels have been hot blued, it is difficult to get a good solder joint repair. And the salt residue is still in there.
There are ways to flush the void area to minimize the salt, but it may still have suffered damage.
And squirting oil into the void doesn't stop the rust.
