The weapons cache is a holotape in Fallout 3.
Location
It is handed over by Three Dog after completion of the quest Galaxy News Radio if the player has already learned the location of James. The holotape points to a hidden weapon cache in Hamilton's hideaway.
Transcript
Three Dog,
Enclosed you'll find the coordinates to the military weapons cache that was uncovered in Hamilton's hideaway. We installed locks on everything we could in there to make sure it stays in our hands. Use it in good health my man, and keep up the good fight!
Copperhead