When I was with the Tropic Test Center in Panama, we did some tests with caching weapons, equipment and ammo.
Burying did not work well. I know a person who buried a couple of rifles as described. He used PVC and copious grease. They ended up as rusted junk after 10 years. (Temperate climate) If you must cache a rifle, find someplace above ground to do it. We found that storage in tree (in tropical rainforest) worked best. If nothing above ground is available, try to find an animal burrow where the storage case has airflow around it.
If it is time to bury, it may be time to use the rifle.
I have a device in my kitchen called a Food Saver. It enables you to seal and safely freeze meat or fish. The machine sucks all the air out and then heat seals the plastic bag. You could seal a pistol but the bags aren’t large enough for a rifle. I can’t imagine why you would want to do that anyway.
Funny. I was scuba diving for lobster in an out of the way place between Portobello and Isla Mamey. “The Sisters” IIRC. 1992 approx.
I found a 2 inch PVC pipe about 6 inches long sealed at both ends with end caps glued in place. Intensely curious when I got home it was with some trepidation I broke it open. I made the kids go inside. It had been packed with grease and a .223 round and a .45 round. IIRC for some reason I felt it had been in the water a year or two from a date inside or something.
The grease had deteriorated badly and the ammo was so corroded I am certain it was useless. I still wonder who did that. Was it one of yours?
During WW2 rice was used to bury things, in Phillipines...not sure how well it worked
I read that when the Philippines fell to the Japs, General Wainright had a Colt .45 revolver that he hid. It was smothered in grease, covered in cloth and hidden way above ground in the crotch of a tree out of view from the ground. When it was recovered years later, it was well rusted up.
On the other hand, Jesse James hid a black powder handgun in a large jar full of rendered hog lard and it survived being buried very well.