Item boxes are a crate type of box which will randomly spawn an item from Civilian to Military Standards, depending on the place you found it. So far, there are three types ("tiers") of item boxes. As with any time guns spawn (as of writing this), they do spawn with random amounts of ammo.
Item boxes can be found in mostly 3 locations/tiers: Whitestone Mountain (North of Rocky Ford) (tier 1, crappy civilian loot)
Hidden Forest (South of Airport) (tier 2, mixed civilian loot and guns)
West Boulder Meteorological Station, Airport, Camp Splinter, Boulder City (tier 3, sweet military loot)
It is not recommended to hop for tier one item boxes. Tier two hopping is viable, but not recommended. I recommend farming/hopping tier 3 item boxes (West Boulder Meteorological Station, Airport, Camp Splinter, Boulder City). Tier 3 item boxes can spawn these items (and maybe more, but these are the ones I have found):
-MAUSER SRG DESERT