As a layman who has made a bit of this stuff, Id say that the metalurgists sound right. The time at welding heat results in carbon migration from steel to iron, possibly other elements as well. That said, if the metal was completely uniform it wouldnt likely etch differently.
It is true that the the twisting process can lead to small tears in the welded ribbands but its my opinion that most of these are re -welded during the spiral barrel welding sequence. The tighter the twist (finer pattern) higher incidence of tearing but also greater homogeneity of the finished product. In a well made barrel, the twists likely didnt result in tearing and the welds are all perfect. This is where the skilled workers made it happen.
Wrought iron is non homogenous and what most inclusions that began in the metal would remain, somewhat refined in size by the process. As Drew has said, higher grades are more refined prior to use in the barrel forging.
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