In modern times we don't think much about runout when working with a barrel. With modern barrel makers' techniques, it doesn't pop up much anymore. 40+ years or so ago, when barrels were formed and then drilled, runout was a big deal. With runout, the bore hole is centered on one end, but not on the other. If the angle is oriented up and down, you can most-times install sights that compensate. If the runout is side to side, you end up looking silly with sights hanging way off to the sides, or the rear notch cut way to one side of the blade. Egads.
If you have a barrel of unknown age and origins, I would highly suggest taking 5 minutes with a caliper to check to see if the bore is centered in the barrel at both ends. I tiny bit of time saves a huge amount of misery later.
The other cause for hitting off from the sights is a bent barrel from the factory (or in shipping). I have had one. No fun. Worth checking with a straightedge before inletting.
I think there needs to be discussion about where to put a bend in the barrel. Do you bend the whole length, or just between the sights (in the case of a rifle)?
God Bless, Marc
Brian Jordan, that is a beauty of a barrel bending operation.