I have a former neighbor, way back when, who planted a whole crop of guns in his back yard. Our neighborhood was in a 100year floodplain. Lots of rain and soft ground. He was in Austin on business for a week and as he's driving home, stops to call the wife (pre-cellphone days). She informs him while he was out, she sold and closed on the house. Buyer paid cash-as is. They now have thirty days to move. Of the 100 or so guns he buried, he recovered about 75% of his stash. The shifting soil in our part of Houston back then moved everything around. With all the building going on in that part of my old neighborhood these days, I wouldn't be surprised to see if one or two of those guns pops up in someone else's backyard blocks away.
He also has a couple hundred acres out in some distant part of Texas. He stored the rest of his collection in an underground cemetary vault (cement construction, need a backhoe and crane to lift the lid off). That is somewhere easily accessible, but nowhere near the main house. The crypt works very well to store all his ammo and guns properly sealed and dessicated. Now that he's retired, his wife is threatening to sell the country property, too.
I always thought, for those who have a septic tank, you could bury your guns along the outer edge of the tank. Someone who has to access the tank is not going to dig up the entire tank, just the access port, so the sides will stay covered. Someone may look in the tank, but not around the edges.
You could also bury it up against a chain link fence. Tie one end off to the fence post so shifting soil doesn't aid in your stash creeping off to the neighbors yard.
Or you could just do like the Terminator did and put all your guns in a coffin at the local cemetery.
Historical data should provide a guide, plan for worse than a '100 year' event. Not that I've cached anything, but I've gave it some thought. 
