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Awesome story!  It's a testament to the quality of materials used back then, and the luck of being shielded by the tree.

Here's my guess as to what happened... 

A miner is out and about.  He's not hunting because there wasn't one in the chamber.  He stops to take a leak/dump/rest.  His buddy then falls into some type of danger... falling down the hill, getting bit by a rattler, etc.  The miner quickly leaves his spot to assist his friend, then turns around and realizes he lost his trail.  In heavy brush, from the opposite direction, he can't find the rifle.  Thus 132 years later, it's found in its original spot.

Or.. he was jumped by a bear/wolf/cougar.  Those are about the only reasons I can think of for someone living on the range to leave their rifle.  If his buddy was spooked by a large carnivore, the rifle would have been the first thing the miner grabbed.

Or... he got caught in the weather, hunkered down and never woke up.
 
Or....   

Great story.  Love to know more of the back history on it if that was ever possible.

I hope they plan to metal detect the area.  My guess is someone brought a few guns out to do some target practice and forgot to pack that one up, (coulda been as recent as the 50's or 60's, maybe).  A lone miner would find a rifle too valuable to lose, I would think.  Plus, it was unloaded.

Cool story!
Always liked the octagonal barrels on the old lever guns and single shots.