Paul, I can't tell you why on a Molecular level, however I have seen this condition occur frequently on trap over and unders. The bottom barrel is fired much more often than the top barrel. Doubles, the right barrel is fired much more than the left. For whatever reason the least fired barrel tends to be prone to more atmospheric corrosion. I am farily sure it has to do with heat and compression of the steel. Just an observation,
Brad
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