The stress rings I'm referring to is when the metal of the barrel has been "stressed" or deformed, the light will cause a series of rings will appear on the inside of the barrel. 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. The rings are a signal to look for anomalies in the barrel, or a better lighting source. The rings can be an indication of a slight bulge that one would ordinarily miss or an indication of the metal of the barrel being worked by either age, fatigue or over pressures.
Sun light is the best light source as florescent lighting often can give a false ringing.
