I have seen large PVC pipes which are capable of holding an M60 machine gun. I usually seen them on sites such as Cheaper Than Dirt. That is what I would use. I would stach some MREs, water, a firearm with ammo, clothing and some medical supplies. I would bury it. Unfornately I don't have a BOL, or other place to go, I plan on buggin in. If I did have a bol, I would bury it at that location. I would like to have one or two at locations enroute to the BOL, but I believe that would be very risky, especially when leaving a firearm.
Extremely risky, and not just simply from the loss of the firearm- if it's found, you have no idea who has it, or what they'll do with it. The loss of it would be costly, of course, but the ramifications beyond that could make the monetary loss pale in comparison.
PVC pipe can be had in almost any size you want, provided you can afford it- even diameters large enough to use as tunnels. Like, walk upright in them tunnels.
In the more readily accessible/affordable sizes, however, you're pretty much stuck with 6" and under, and even those aren't cheap. I just bought a 10 foot length of 6" schedule 40 PVC, and it ran me 45 dollars- all I needed was about 2 feet of it... (anyone wanna buy 8 feet of pipe?
) FWIW, tho, here's some information that might come in handy, which could otherwise be misleading to a buyer not in the know- pipe sizes are based on INSIDE diameter up to, and including 12" pipe. AT 12" size, there is both 12"ID and 12"OD (which is where it starts becoming confusing), and after that size, it's based on OD (Outside Diameter). To find the inside diameter (sometimes it's a tight fit for your gear) determine the wall thickness, multiply it by 2, and subtract that number from the OD of the pipe.
If you plan on using pipe for cache containers, try to make them look like they belong where they are, and could possibly end badly if disturbed. Since PVC is often used for sewage applications, no one is likely to mess with what appears to be a sewer pipe- there's no gain to be had from it. I've seen PVC used as the shell of car barrier posts, filled with concrete, and set into the ground. As a cache, hiding in plain sight with something like that could be a good idea. On the other hand, it's also less secure if someone DOES mess with it.
A cache is as much about camouflage as it is security against the elements, especially when the container is small enough to move if discovered.
My first two warning shots are aimed center of mass. If that don't warn them I fire warning shots at their head until they are warned enough that I am no longer in fear for my life.