Well, here’s a little anecdote. Once, years ago, when scuba diving about 60 feet down I encountered a piece of 2 inch PVC pipe about a foot long with end caps glued in place. I took it home figuring it might just contain something of value. I cut it open and the end caps were still very securely glued on and in each end was a couple inches of what looked to be Vaseline. Inside were a couple of plastic zip lock bags and each had a bullet of a different caliber. .45 ACP, .223, etc. I have no idea how long it had been down there. It was obvious I had stumbled across somebody’s experiment. I don’t know how in the world water got in there but it had. There was loose water inside and all the bullets were hopelessly corroded. Don’t ask me how it got in there but it clearly did. Sure, burying it in the back yard may be fine if you live in Arizona but where there is ground water, like almost everywhere else I wouldn’t be certain. Also you have to try to bury them under sewer pipe or something to fool the metal detectors. IMHO, burying stuff may sound practical but I doubt it actually is.
PVC is impermeable to water, so the leakage was due to the “sealed” end caps. However they were sealed it wasn’t good enough, and I would suspect seasonal temperature variations contributed a lot as the pipe and caps would expand/contract repeatedly.
Maybe we should take a lesson from the ancients. We still find sealed clay pottery jars in the bottom of the Mediterranean that haven’t been compromised thousands of years later. The Greeks and Romans sealed those with ceramic caps and tree resin/pitch, possibly with wax, lime or other simple compounds added.
There is quite a bit of difference between burying something in the ground - above the water table - and actually in the water.
Anyone that lives in an area with at lest some variations in the terrain should be able to find an area that will stay dry.
Add to that properly preparing the pvc vault - pipes can keep water OUT as well as in.
What other choice do we have - let the commies confiscate every gun?
The pressure at 60 feet under water was part of the problem—maybe the whole problem.