I was reading Drew's initial work and thought about the 737 Aloha Airlines plane that landed with the front third of the top of the fuselage ripped open---was traced to corrosion and the fact that each time the plane takes off and lands the fuselage is stretched open and closed. This slight movement over time "work hardens" the metal and makes it brittle. I would guess that a shotgun is the same way--each time it is fired the barrel gets a hoop stress as the metal tries to expand. That might be significant in an old barrel