If you truly are going to bury them, bury the pipe vertically in the ground, for the smallest footprint possible. If possible, try burying them in or near a junk/scrap yard that may be close by.
Yep, I remember those tips. For the pistol and for small boxes of ammo, I was also thinking about putting in an oxygen absorber and then putting all of it into a Foodsaver (vacuum sealer) bag and sucking out the oxygen. Waterpoof and airproof.