I have used cache's on many occasions.

Being nomads, we frequently need to leave things securely in one area that are forbidden in another. Longest I have left things for real is about 7 months, although I have done some small tests that have been 3 years.

The best reference I have found is;
TC 31-29/A
Special Forces Caching Techniques
U.S. ARMY SPECIAL FORCES
CACHING TECHNIQUES
U.S. ARMY
JOHN F. KENNEDY
SPECIAL WARFARE CENTER

Links change so I won't post one. Just google it.

If storing a weapon, I usually remove some vital part and put it someplace else. Although this has caused me to have to buy a new bolt for a semi-automatic when I failed to properly record the location data. Lesson learned....

I suggest you try caching something, perhaps in your own back yard. Doing is the best way to learn, and it is not as easy as it sounds.