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The centers are used to set up the steady rest either on a spider (octagon barrel) or direct on a round barrel, then the crown is cut. If you ever have occasion to set up a barrel in a head stock of a lathe, dial indicate both ends to center ( without the breech plug installed) run at a slow RPM and look down the bore. It will at times look like a jump rope the bore can be so crooked. That is the reason for orientating them in the vertical plane so the sights can more easily compensate for the runout. That is also the reason deep hole boring well/accurately with minimum runout is the real secret of a good barrel with the rifling itself being the next most important, in my opinion.
