The survivalists that I was dealing with in southern Oregon prepared their guns for long-term storage (burial) by separating wood, plastic and metal parts, oiling/greasing the metal parts and vacuum-packing the gun parts, waxing the wood parts, vacuum-packing the plastic, optics and the ammunition separately, packing all of this in PVC pipes, sealing them and putting them in their carefully selected hides, usually vertically in deep post holes. Some liked to flood the tubes with nitrogen, like they did with food caches.
All I can say is that the process worked.