Here is a close up of Tom's barrel segment. I suspect there was some carburization at the butt weld causing the loss of detail in the contrast
Pattern welded barrels were fabricated with (usually) 3 tube segments; thicker at the breech. The butt weld is clearly demonstrated, and my only guess is that the mid-barrel section had different iron and steel (and therefor different coloring.)
Picture Trail is ceasing operation in 6 weeks and I have to replace every image on the DamascusKnowledge site
but for now more examples are here
http://www.picturetrail.com/sfx/album/view/18015717
One of Bruce's examples of mis-matched tubes segments
Ken Marburger refinished these barrels. The pattern is pretty close but the coloring is a bit different.
