Checking barrel straightness in a swamped barrel
Concentric rings down the bore indicate a straight bore, given proper lighting. If the bore is straight but the outside looks asymmetric, the concern is how far the outside diameter is off relative to the bore, because it can leave one wall thinner than the other. Often the difference is not significant, but it can be. When a barrel is significantly off, Kibler's scraps it and cuts it open to inspect wall thickness.