Just my opinion, not backed by any experience:
It seems to me that sealing a gun inside a rigid container (such as a PVC pipe) in a vacuum only protects it because, by pulling the vacuum, you have removed the air, along with any moisture that may have been in that air. But once you do this, then you have about 15 psi of pressure on the outside of the container and if there is a leak you'll get air (around 20% oxygen) coming in, plus whatever moisture may be in that air. I would be more inclined to fill the container with dry nitrogen, maybe even at a very low positive pressure (to minimize stress on the seals) so that any minor leakage would be nitrogen out, not air in.
Also, silica gel absorbs water vapor from the air, which is good, but the air is still about 20% oxygen so, being paranoid as I am, if I used silica gel in a sealed container (without replacing the air with nitrogen) I would also add some packs of an oxygen absorber.