Also, your xrays of the damascus barrels didn't seem to show a network of weld joints, but rather the two components became a mono steel. I would suppose if the TIG welder thought they formed some embrittlement or other worry, they could stress relieve the barrels.

That appearance of the longitudinal barrel failure on the Parker sleeve gun might be a reason why the damascus smiths turned the long grain of the barrel's parent materials to right angles with the bore. I know it's not related, but that's how I would guess wrought iron, or questionable roll mill steel, might fail if the long grain followed the length of a barrel, a different intent from construction fasteners, axles, etc.