Sherman Bell did considerable testing on NFB vs. BP loads and his test, most of which relied on strain gauges, but some of which relied piezo transducers, iirc, proved out that NFB loads produced less peak stress. Also iirc, choice of primer was important, with too hot of a primer causing a less than critical, less than BP peak, but notable spike in the early phase of ignition. You can read his results since he published them in an article in Double Gun Journal.

On damascus barrels, he tested to failure a whole bunch of damascus guns and some early and later fluid steel barrels. These were all shotguns, iirc, and the damascus generally faired better than the fluid steel. Often, the action was giving away and ribs seperated while the barrels hung in there, some bulging. Some of the guns' actions or barrles were in pretty poor shape to begin with, and none was pristine (as you might imagine for a test to failure project), and he was using modern SAAMI proof matching loads. When he could, he continued shooting til a bearrel blew, some did, but other times the action failed first. You can also read of these tests and results in Double Guns Journal.

Prior to reading his test, I figured that if the barrels of a shotgun or rifle looked alright and checked out all right for proof dimensions, etc, with a good gunsmith, I'd try them with a lanyard and NFB. After reading his tests, I'd use them without hesitation if the passed, say, JJ's inspection, just the same as vintage fluid steel barrels.

JPK