...I believe it is caused by the light undergoing a slight shift when it hits the metal that has a different molecular structure then the metal beside it...
You mentioned that this may be a sign of deformation in the barrel but you seem to feel the tube is true. I think you are seeing deformation, but if the barrel is true, I don't know if differences in surface appearance could lead to significant conclusions. The book that was mentioned might be worth checking. You may not prefer one of the authors, but I think there is meaningful contribution by the coauthor.
