If you are preparing for any contingency, whether simply surviving a disaster or getting ready for the collapse of civilization, there is one ammo you need above all others.
It is the .22 long rifle (.22 LR).
I discovered the .22 rather late in my shooting career. Growing up in New York City did not afford me the simple pleasure of going into the woods with a .22 like millions of other youngsters. Our first firearms as teenagers were the M16A2, M249 SAW and M1911, courtesy of the US Marine Corps.