I thought I was pretty clear. Go hiking in hilly, rocky terrain like this. You'll see rocks leaning against trees and they leave a distinct impression on the bark. Most of those rocks have only been there a season or two after rolling downhill. Occasionally you'll see a rock totally embedded in the side of the tree, those have been there much longer. Don't you think the bark immediately adjacent to the rifle would have grown or expanded over 100+ seasons? Don't you think the bark under the steel barrel would behave even the slightest bit differently? The result would have been an impression or distortion of that part of the trunk. There is nothing visible. I suspect it had been there no more than one or two years.
As I said above. I think someone found it nearby and then leaned it against that tree.
Things grow very, very slowly in the desert.