Providing that Damascus barrels are in good condition they are just has safe to shoot as any steel barrel .What happened to the gun in question was a obstruction and could and would have happened to any gun . I have shot Damascus barreled guns for 61 years but the advantage we have hear in the UK over American shooters which is the proof house.There are many Damascus barrel guns go for proof every week mostly early breech loaders and 80% are proofed for nitro.
Feltwad
There is nothing wrong with damascus as a barrel material.
HOWEVER, no old barrel can be considered suitable for shooting. To many variables. Material, quality of the work, abuse in the past, overloads, pitting barrels with invisible damage from being dropped or banged/bumped which can break or weak damascus barrels.
On double guns, according to Greener. A bump on one barrel can break a weld in the other.
This barrel could have been damaged at any time in its long history.
Note the flaw, ding or dent just behind the break.
The barrel is pitted and this is another way to promote failure. Then there is the possibility of some small flaw in a weld that finally got tired of the abuse of being fired. Perhaps the barrel work hardened over time. Has happened in the recent past with modern shotgun barrels.
The corrosive "replica powders" can eat "crawdad holes" in steel if it were ever used in the gun it could have been the cause since it is very hard to get stopped once started.
I submit that it was not a wad, there is no defined ring, but simply a case of the barrel finally giving up and breaking.
People need to understand that they can shoot old guns all they want. But they need to have an understanding that there is a level of risk involved in this activity. This allows them to make an informed choice.
Dan