It's interesting. One thing I wonder is why not just immerse the gun in oil? Put the gun with a lot of gun oil inside a plastic bag. Then put that inside the PVC tube and fill the PVC tube with motor oil and cap it. Even if the plastic bag leaked it would just mix the gun oil with the motor oil, shouldn't be a big deal.
Another thing I thought about was the idea of placing your OWN "tree stumps" in the form of metal rebar. If the ground isn't too hard and rocky you could drive a couple 6 foot long rebar's into the ground and leave them sticking up a little, bend the tops over so no one hurts themselves. Two of them with a line drawn between them as the base line. Then mark the gun with a perpendicular imaginary line from that some distance away, could be 78 feet for example. Far enough that no one is going to be able to use your baseline to find anything unless they know the 78 foot distance you used. You also don't have to draw the perpendicular from the midpoint, it could be at the 5 foot distance from the southern rebar (lets say the rebar is 20 feet apart. You would also measure the location from other natural landmarks, but the rebar is going to be easy to find later with a cheap metal detector. And of course, you would also document the rebar's locations relative to natural landmarks. This makes it possible to have a several way redundant relocation method without the metal detecting part being able to give away the REAL gun location.