Ed, a lot of Double guns, both Damascus and Fluid Steel, had their barrels joined with corrosive flux. It was mostly used for the initial step of tinning the barrels and ribs prior to actually soldering them. That's because corrosive flux is generally more forgiving and easier to get good results. Usually, the corrosive flux residue was removed prior to joining with soft solder and a rosin o,r other non-corrosive flux. Flux does not mix in with the solder, so any residue lies on the surface or flows to the edges. It does not get trapped under the solder. You've apparently seen what lies between the ribs of at least one old Parker, but a lot of others are pretty cruddy in there. It isn't normally any problem unless there is enough active corrosive residue left to eat through the tinning and then work on the barrel tubes.
In addition, many gun makers tested and advertised their Damascus and Twist barreled guns as being safe with smokeless powder, after it was introduced. Proof houses certified that many Damascus and Twist guns were Nitro Proof safe, and they were safely and successfully used with factory smokeless loads for a long time before attempts were made to render them obsolete and unsafe. Remember, the Damascus blow-up tests done by Sherman Bell, Buck Hamlin, and others used barrels that were sacrificed because they were already quite pitted and damaged... but they still held up under rather extreme pressures.
A barrel does not know if the pressure produced by any load came from Black or Smokeless powder. As long as the barrel is strong enough to withstand the the pressure produced at a given point in the pressure curve, it will be fine. Use a heavy enough load of black powder, and that too can bulge or blow up a barrel. As I recall, the English Proof Houses continued to do Proof Testing with Black Powder Proof Loads for quite a while after the introduction of Smokeless Powder. So any that failed proof did so due to the use of Black Powder.
You may recall the one Blow-Up Thread posted by the Preacher that involved a fluid steel barrel on an L.C. Smith. There was a pretty obvious inclusion in the barrel at one of the thinner points under the chambers. Modern steel is very good stuff in most cases, but defects like slag inclusions and rolled-in scale still happen, and mostly go unnoticed until a failure occurs.
Many shooters will continue to shoot 100 year old Damascus and Twist shotguns. And many people will continue to drive 100 year old vehicles that lack seat belts, airbags, or any crash protection in their design. We just have to understand and respect their limitations. Every shot we fire from any gun unleashes thousands of pounds of pressure only inches from our eyes, so it's all a leap of faith.
