I've always just used a paste flux and solid core lead/tin solder. Both for the initial tinning of the surfaces and then the actual soldering after positioning and clamping everything together. Never had a complaint.
Most any flux that doesn't seem to work well is often because the operator over heats everything and burns the stuff. That destroys the protection from oxidation the metal surface gets from it during the soldering,
Get the surfaces clean, well fit, parts evenly tinned and most any decent flux will give you good results if you don't over heat things.
If you tin it with acid flux then clean and neutralise the acid then use rosin flux to put it together I also use a natural fiber brush to get a consistent tining.
When I build double barrel rifle barrels and attach the bottom and top ribs, I use a tinning paste that auto body shop workers use. It can be purchased in lead/tin as well as 100% tin. After you apply it you MUST clean it absolutely clean to remove the acid and then you additionally wash it with backing soda several times.
Travis, To answer your question, yes, rosin is a neutral substance, it is a good flux and does not cause corrosion. It makes no difference as to steel or damascus. The only down side is it is very heat-sensitive and chars easily, so tinning has to be dove with a soldering iron, electic, is easiest, about 600 Watts. I have never used a non-electric iron, but I would imagine one heated on an electric hot plate would work, but you would have to keep it up to temperature to melt the solder and and not over-heat to tin the barrels. You need a 600 Watt iron, they are available used on ebay, but make sure it works before it is yours. Just my opinion, but it is far easier to tin the barrels and ribs using an "active" flux, like Oteys No 5, available in home centers, the solder flows quite easy using it, but you need to remove the salt and tallow that the salt it is mixed with; it is mixed not compounded. Washing the parts with mineral spirits and then washing with a strong detergent like Superclean diluted about 4:1. To demonstrate the need to wash all the flux off after tinning, try tinning a piece if steel and just let it sit over a few days. One of the active salts in some fluxes is a component in some rust blue solutions.
Final assembly is with rosin flux. Taking apart a good quality barrel set often shows no corrosion at all, and the void between the barrels has ben sealed up for years. And the rosin cleans up with alcohol and steel wool.
He made an error when he addressed hot bluing a soft soldered set of barrels. He stated that the heat from the hot bluing melts the old solder. Not so. The resulting corrosion from the salts eats away at the solder joint, but the temps attained in hot bluing aren't as high as needed to melt 60/40, which is 361 degrees F. Hot bluing rarely exceeds 260-280.
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