"My opinion so far is that the firing pin hung up. Dropping the pin on an empty chamber I could hear two distinct "clicks." First a faint one, then a more distinct one."
I agree.. Sounds like first the sear/ striker spring tripped then the striker hit at it's forward point. The shiny spot would induce me to think there was a bit of grit or contact in there - maybe a metal shaving or sand grain, kink in the spring, slightly bent striker shaft or a rough spot. Or some sticky gunk as mentioned by others. If the spring is a bit weak, it doesn't take much.
If your cleaning seemed to solve the problem, it was the latter, maybe with some help from others.
I use only dry to touch lubes (Eezox or Corrosion X) for these applications, extra strength springs, and never ever store the rifle cocked - always with the striker spring tension relieved by dry fire.
I may have had a similar problem(s) somewhere in the past......
Moisture frozen in there can do the same thing, but you said it was above freezing.
