So, depended on how it was leaning against what. Doesn't look like a position that necessarily the tree, growing slowly under rough conditions, would engulf the rifle.
I’ve been saying this since it was first found.... The rifle was probably placed against the tree in relatively modern times. If it had been leaning against the tree for 100+ years there would have been substantial interaction with the bark growth and there is no trace of that. My theory is that the rifle was found by someone in a more likely place, like at the bottom of a cliff or down a ravine where it was first dropped. Someone (who doesn’t know much about firearms) picked it up, possibly carried it some distance, placed it against the tree, and continued on their way — probably within the five years preceding its discovery.
That's out in the desert and actually the barrel is in pretty good shape, but I've seen stuff near mining camps dating back to 1900-1930 with pretty much the same patina, leaning up against the tree trunk. Tho I did find an model-t axle embedded in the crotch of a tree once. It was about as rusty as an anchor chain left out on the waterfront for a few decades, ie almost completely converted to rust. But I've found stuff part buried by accumulated detritus with negligible patina, obviously untouched for decades. Just depends on the locale and particularly precipitation.
When we went camping out west in the 60s-70s we also found a lot of tin cans high up in the trees. My mom said it was once the custom was to use tin cans to boil water and just set the cans in the tree for the next camper to use. If the tree didn't get much water or sun the cans would still be at eye level after 30-60 years, but if it was relatively wet you might spot the remains of the can in the treetops.
So, depended on how it was leaning against what. Doesn't look like a position that necessarily the tree, growing slowly under rough conditions, would engulf the rifle.