Buried Firearms aren`t uncommon in New Zealand and Australia. Between my Father, his brother, my brother and myself, we have 19 weapons buried in 7 cache's on 2 family farms, and at least 5 of my friends (that I know of) have weapons buried in thier back yards.
Due to
Incredibly restrictive gun laws that came into being after the 1990 Aramoana Massacre in New Zealand and the 1996 Port Arthur Massacre in Australia, All full-auto and semi-auto Rifles were affectively outlawed.
As well as hi-cap mags, most Handguns, shotguns with more than 3rnd mags and any rifle mag that will hold more than 10 .22rnds, Regardless of the rifles calibre!
A length of 150mm plastic pipe, with screwed on endcaps, will comfortably hold 2 weapons. Pack it full of grease, Shove it into a 225mm plastic pipe, pack out with more grease, attatch endcaps with epoxy resin/fibre glass, wrap the whole lot in PipeWrap anti-corrosion tape and you can bury that sucker anywhere you please for as long as you like.
Links to Aramoana,
http://en.wikipedia.org/wiki/Aramoana_massacre
and Port Arthur. For anyone interested.
http://en.wikipedia.org/wiki/Port_Arthur_massacre_(Australia)