Im sure everyone already said 7.62x39, and if they didnt they should have.
If your SHTF situation is something along the lines of burglar breaks into your house, then any caliber that can kill a man in one shot to the chest is good. Well sort of, in an apartment you might want to have a better round so as to keep from shooting through walls.
However, in a WCS (Worst Case Scenario), WCS meaning an army invading, a large portion of the populace rioting or revolting (whether your the portion or not), almost absolute or absolute societal breakdown, etc etc. In a WCS, whatever army or protectorate your country has is unable for whatever reason, to help. That means no Police either. Typically a WCS is a somewhat surprise situation, or a long degradation situation.
Well in a WCS, the clear winner IMO is a AK-47 in 7.62x39, for mainly three reasons.
1. AMMO AMMO AMMO, let me say that again, AMMO AMMO AMMO.
-In a WCS, you dont get resupllied with anything, your fighting a One Man or Small Squad war, you and your fellow people (should you have any), must procure ammo and supplies through theft, killing, or scavenging. In America your going to find a little bit more 7.62x39 then .223, unless you come across US military places (depots/bases/etc.). So in America your choices are typically 7.62x39 or .223. However, most countries aggressive to the US use 7.62x39, not all use it as there primary, but many do and all of the Aggressors that have it will use it in some fashion.
So if China invaded (however implausible due to their craptastic Navy, and the US's superior military technology and training. Also, and above all else, Nuclear Weapons) they would have a large quantity of 7.62x39 and little to no .223.
In the world, there is more 7.62x39 then .223.
2. Durability and Reliability
- This is a given, in a WCS situation without any supplies (ie, no cleaning supplies), you just wouldnt want a M16 or other rifle that needs to be cleaned alot. Unless everyone is carrying around cleaning kits, you dont want a weapon that needs to be cleaned as much as an M16.
Go with the "Mud Crawling, Sand Blasted, Gunk Busting, Ass Kicker"
3. Availability
- Im done explaining things, this is a given as well.
AK in 7.62x39 = good for just about anything