I don't know if they still do it, but years ago, Douglas advertised their top line of barrels as XX, and I believe they added Supreme Air Gauged to that.
In the Douglas literature, it stated that these barrels were some of the few that were actually drilled straight and didn't need straightening. Apparently, it is very difficult to drill a deep hole in a bore perfectly straight, and a lot of them (most) needed straightening.
I also remember Douglas literature stating that the gunsmith, when countouring the barrel, needed to turn them between centers, so that the bore remained in the center of the barrel. I have always done that, and I have never had a barrel that I turned with an off center bore.
As a poster above stated, the result of an off center bore would be the result of the final contouring. Some barrels are formed and turned before boring, and this could result in an off center bore.
Turning between centers should center the bore in a blank that is bored off center.
My barrel work has been limited to Shilen, Hart, McMillen, and a few factory barrels. I have never seen a barrel with an offcenter bore.
