[Oatka]

It is anyone’s guess when it was placed there - any time between 1883 up to 20 or so years ago. My wild can’t-prove-a-thing says after WWII. There could be clues if the gun was loaded - the head stamps, bullet composition (lead/jacketed) and/or type of powder (blackpowder/smokeless)
would narrow the date range.

If it shows up on a 4473, that brings it into modern times.

[Robe]

On anther note, what is that to the upper right of the trigger.?
It appears to be a metal bar with a ring in the end,with maybe a strip of leather attached, and if you enlarge , there seems to be two holes in the middle, with the far right embedded in the tree.
It sure looks like a bridal bit.
I have heard of skeletons of horses still tied to trees/bushes, the horse just stood there til it died, kind of a ghastly thought.