An interesting thought experiment, and a very interesting real-world effort by Charles Wood (the backwoodshome article).
I like the PVC plan, but a buried PVC pipe setting off a metal detector is pretty obvious.
Why not bury your "time capsule" in/with a chunk of scrap metal? Do the PVC and bagging plan, add plenty of desiccant, purge with nitrogen or carbon dioxide, seal it up, and plunk it into a junk metal container, preferably one that appears to be something unpleasant, discouraging tampering. You of course don't want something so nasty-looking that an unwanted finder calls the EPA or Sheriff.
Perhaps a 55 gallon drum labeled as "asbestos waste", and stuffed with industrial scrap and sawdust?
Or re-purpose a pet casket and label it "beloved Fido, faithful companion", adding a large metallic latch and lock to explain the ferrous content?
Acera said:
I would not bury all my collection, however some that pose a particular interest to the anti-gun establishment is a viable option.
If one was burying such things, the older, simpler, more durable ones might be the best candidates. WWI/II relics, old revolvers, non-semi-auto rifles, survival rifles, etc etc.