I'm a believer that barrels drilled to make the bored blank, reamed to uniform the bore dimension then rifled to put the grooves in are best centered in the barrel when it's all done on the same machine that spins the barrel. The gun drill, reamer and rifling tools all align on the center of rotation. Then the outside could be turned to profile desired with the bore spinning on centers at each end.
This is how the most accurate barrels are made these days; either button or cut rifled ones. No hammer forged barrel to date has equalled what the best of these do. . . .at least as far as I know.
But this is microscopically off thread topic......