Not having seen these barrels, I am only 98% sure this is not a crack. Pressures sufficient to cause a barrel crack would have ruptured before going down the entire bore, and cracks in damascus barrels do night form as you are describing.
There are many good NDT labs in most metropolitan areas, and can perform one of several exams which will be more conclusive than blind internet guesses.
Also, I would doubt a plated lead pellet would do this, by itself. If a pellet were to leave a trace down the bore, it would be so slight that any good bore scrubbing would remove it.
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