I prefer water and food before that both..... 
Well... taking in to account the fact that an average human being requires 0.2 cubic meters of water every day, you might need to store some 14,600 cubic meters of water, if you want a family to survive for 50 years without any supplies from outside. It will require a lot of space. And the case with food is even more trickier. Most of the food items will get damaged after 6 or 7 months. How will you store them?
Canned meat can be stored a few dozen years. But I guess if that was a question, you should better care for the safety and preservation of your ammo...