If you have legal access to property, you can at the present time bury whatever you want--guns, gold, whatever.
If some sort of confiscatory law were passed, and you didn't dutifully dig up and turn in said firearms, you would THEN become an outlaw.
Best would be in a location not associated with you, personally, but to which you'd have access. That's for you to figure out, not me. If somehow the firearms were found, you'd not be Mr. Suspect.
Method? Nothing wrong with the preservative and PVC pipe idea.
Given what's going on in the world, I don't see Congress or any state trying to pass a confiscatory law. For all that the numbers of us, the activists, are relatively few, a confiscatory law would then gore the ox that is the greater majority of non-activist gun owners--and that's some 40 million households.
IOW, forget the tinfoil hat stuff.
Art