Well... guess I'll add my 2 cents worth...
I've read through the varied responses, most the guys have valid points and selections!
Just depends on where you are when it hits, and how hard it hits...
... Nothing like but here goes...
I've setup my "SHTF Prep" with some basic standardizations...
45ACP For all my handguns and my "quick response" Carbine, Just need some bigger mags for the carbine...
308 For my "kick ass" surburban and long-gun with 1.5-10x30... SAIGA of course...
12GA Shotgun for close in work, not a Saiga but nice, reliable, and damn accurate.