we have an apples and orange situation here.
Yes, modern loads are based on the "dram equivilent" loading BUT using the smokelss powder the pressure time curve is LONGER.
Blackpowder loads have a shorter pressure time curve.
Look at the barrels they are heavier around the chamber for a SPECIFIC reason,
to withstand that shorter curve.
With modern loads that peak/ decreasing pressure spike is further down the barrel, were ii is thinning.
Damascus barrel construction (of iron /steel) or (steel/steel) increases the area
for pitting the form and weaken the barrel.
[I certain yiu all have seen used guns in racks, that HAVEN'T meet a cleaning rod???]
If you are set to shoot an damascus / twist steel barrel, then consider contacting Griffin & Howe for sending it to Britain for reproofing and FOLLOW their advise.