In a panic situation like the end of 2013, a non-popular caliber sidearm like a 38 super, 9x18 mak, 7.62x25 tok, or the similar was nice to have. That ammo was everywhere while 9x19, 40, 45 was scarce. Revolver ammo - if you like shooting them - was available for a while too.
Of course it was nice to be eyeballs deep in 223, 762x39, 308, and the popular pistol calibers as well.
A stockpile of 556 mags for a quality AR with tons of ammo was very reassuring for me. I am very comfortable with a 556 AR though some others don't like them.
If you're an AK guy, you could go with 762 or 545 spam cans, mags as well.
Panic situations really force you to think - what do I need vs. what are nice to haves. To be stockpiled in ammo and mags with a reliable firearm was the goal.
For us city dwellers, you probably can't go wrong with a mossy 500/590 or Remy 870 with a metric fuck ton of 12 buck either.
Just some thoughts and memories...