I was shooting my Winchester 1500 (basically its a classier version of the more common Winchester 1400) the other day and after a few shots of just some federal target loads I noticed a slight discoloration appear on the barrel. It didn't cover a large area and it only appeared on the underside of the barrel. It was a cool day out both times I shot it and I know the gun gets hot after shooting but could this be a sign the barrel is weak at that point? It usually fades as the gun cools down and I can wipe away the rest with my gun oil rag. I posted a picture of a barrel that I found online and circled exactly where the discoloration appeared. I'm hoping this only happened due to the outside temp being in the 40's vs. the gun getting really hot really fast due to the discharging of 12 gauge rounds. However if this is a warning sign of something more serious please let me know.