I think all damascus barrels with minimum wall thickness of less than .040 are extremely dangerous and should be left to those who collect them as non-firing relics. And since they are so useless they are terribly overpriced. It is common knowledge that when they were built they were full of flaws and over the past century or so the metal has deteoriated and weakened further. They're nothing but time bombs waiting to remove useful parts of your body.
I do not agree with the .040 minimum on all Damascus guns in order to be shootable. I believe the decision on shootable Damascus is both thickness and bore condition. If relatively unpitted and clean (not honed enlarged) and .030 or better it is shootable and I would be ok with less given the quality of the barrels and condition (this is rare)
On Fluid steel I use the same rule of thumb and go with .025 as the minimum.
I have shot thinner without issue, but did not feel comfortable.
Note on minimum thickness barrels I always stay with low pressure loads (which I define as Sub 8,000lbs PSI) on then 12 gauge Damascus I use sub 6000 PSI 1 oz loads)
In the end shooting old guns is in reality a gamble which one contols the odds some with good judgment, but not completely in terms of what you are willing to shoot and how far one is willing to push it. Disgression is key.
