I have set up, staffed and managed Quality Control and NDE Departments in the Nuclear Industry. I assure you that i am familiar with just about any NDE procedure including UT. The one thing that has not been metioned in this discussion is the requirements for 'Passing' an NDE exmaination. These requirements are rigerous and refer to porosity size, number of indications, linear defects among others. A 'Tech' does not give you an opinion, he is not qualified to make a determination of what is safe and what is not safe but tells you what is there and then the code tells you what is acceptable. Interpretations are done by qualifed Welding and NDE inspectors to written criteria.
In all the conjecture I have read, there is no rigerous standard for what is safe and what is not in testing a damascus barrel with NDE. Coupled with the manufacturing methods and materials of the day, there is no way to determine if what you find or don't find is safe. If you think that you can make that determination for yourself simply purchase the NDE materials and do the testing. Dye Penetrant (PT) is very simple, spray on the penetrant, wait, wipe it off and then spray on the developer and look for red indications. As I said in another post, it will be a 'crap shoot'.
I understand the romance with wanting to safely shoot these old shotguns. One of my first doubles was an Ithaca 'Long Range' Double manufactured by Western Arms. I shot a few doves with that gun but research showed the reciever to not tensile steel but a cast type of steel. This type can be subject to brittle fracture. I sold the gun to my dealer. I can not predict the future.
Perhaps it my mathematical and physical science traning but I examin what I am doing and make a determination of what is safe. As the Quality Assurance and Nuclear Safety Manager, that is what I am trained to do i.e make safety decisions which adhere to Code, State and Federal Law and if there are no standards, make them in a controlled and rigerous manner. So I have no problem in making the determination, that you will not obtain any data that will allow you to make any objective determination about your barrel or barrels. -Dick