I suspect the answer to these perennial questions is "all of the above". UV light degrades certain hues without mechanical contact, as pointed out by the display case observations. That is a simple enough experiment to perform. It implies a photochemical reacion.
I don't think this topic really needs to fall into either/or categories. Physics vs Chemistry.
The films composition and thickness can be changed by many things. It might be interesting to fire a shotgun a few thousand times in the dark, jigged so as not to touch the reciever, and then observe any changes. Chuck's hinge-pin discussion reminds me there's plenty of energy at firing to bend the action bars, and possibly disrupt the films. This would demonstrate brittleness I believe.
Then of course, there's just plain ole' oxygen, anxious to join it's Ferr(ous,ic,ite)brothers on the road to entropy.
Someone with a kiln could easily do experiment 1 at no significant cost.
