The Autumn Double Gun Journal has part XI of Sherman Bell's 'Finding Out For Myself' series. He took 15 twist and damascus brl 'wall hangers' and subjected each to the Remington-brand industry-standard proof load which Tom Armbrust reported as having an average velocity of 1378 and pressure of 18,280 psi All guns were well used if not abused and some had deep and extensive pitting. The oldest was a 1877 Parker and 'youngest' a 1913 LC meaning some of the guns had more than 100 years of hard use prior testing. Most had 2 5/8" chambers but some 2 3/4". He reported in detail on 8 guns and will give a report of the other 7 in a later article. Bottom line: NOTHING HAPPENED TO THE BARRELS Some guns were made MORE off-face than they already were and some cracked stocks cracked more but to quote Bell "...the old guns were not exactly in shock and awe of modern proof loads."
I'll just quote some more of his remarks:
"Only a fool would presume that the results of testing one set of Damascus barrels prove that all such barrels will take the same amount of abuse."
"I believe a gun should be subjected to pressure levels commensurate with that produced by ammunition made during the time period of its manufacture." (This may well be the take home message.)
"...we have found out that 16 (including his one earlier damascus experiment) Damascus-twist 12-gauge shotgun tubes, in various condition from pretty good to absolutely horrible, underwent no measurable dimensional changes and no additional damage when fired with modern, very high pressure, definitive proof loads. Not only did they not blow up, they did not split or bulge or change dimensions in any way."
"Can we extend the results of this proof testing to predict that carefully inspected and well cared for Damascus-barreled double guns such as your or mine, are all absolutely safe to fire? No, absolutely not!"
Like Tom, I choose to shoot damascus guns with low pressure loads ONLY after measuring wall thickness and examination with a bore scope for pits/internal defects. My choice and my fingers (but unlike Dr Bill, as an internist and preacher, I only need my increasingly squishy brain )