I can see the argument from both sides, but if something happens that you loose your main firearm, would you still have access to a shovel, or a way to dig it up fast enough to matter?
If one absolutely HAD to bury a firearm all metal surfaces would require a thick coating of Cosmoline but removing it can be a nightmare. I have purchased many surplus weapons over the years that have been stored with this and the only thing that worked well in its removal was gasoline. I have heard, but not tried, WD-40 will dissolve it but not sure what it would do to an old wood stock or the finish.
Also if said firearm is to be placed in a PVC container, prior to sealing, it would be best to include a military grade de-humidifier as well.
I am not sure I would personally bury a firearm for a SHTF situation since it wouldn't be readily at hand when you need it.
Are we talking about literally burying them into the ground? Will you then keep a shovel with you at all times? Or have a backhoe as your daily mode of transportation? Cause if we have a drought like we've had these past 2 months, it might be pretty close to impossible to get them out of the ground without the right tools.
*edit* first few pages didn't mention this. Guess I wasn't fast enough to be the first one with this joke.
The way I'm doing it, once it's buried you don't have to dig much to get to it. Once it's sealed in the PVC, humidity will not be an issue. Nice pack of do not eat, you'll be fine. If you've sealed it properly, no more moisture will present.
I don't think there should be a concern about a shovel. This isn't meant as a "I have to have it right now!" sort of deal. There will be time to find something for which to dig. If there isn't time, then it really doesn't matter anyway. People bury food and other supplies. Hiding by burial is a time honored tradition. My wife plans on hiding my body by burial and isn't worried if she has a shovel.