Ammo is easy to find, something that must be considered in a SHTF situation
First off, we don't do SHTF or TEOTWAWKI fantasies here on TFL. They can be entertaining, but there are other places on the web to play those games.
That being said, we do entertain serious discussion about realistic situations where the normal rule of law and order of business could be temporarily interrupted. Fire, flood, earthquake, hurricane, volcano, or people rioting like chimps, and their aftermath are acceptable topics. The base understanding is that law & order WILL return. No post apocalyptic fantasies, please.
so, does a certain caliber being easy to find matter that much???
I don't think so. Even if one is cut off from buying ammo the normal way for month (which I consider pretty extreme) is that going to matter??
Yes, if you don't have any ammo of your own, BEFORE the disaster. But if you don't have your own ammo, who's fault is that??
Don't count on being able to scrounge or salvage ammo, because, #1, its called LOOTING and people get shot for that...
#2 what makes you think any place that would have ammo will be deserted, and won't be picked clean well before you get there?
(if you get there early you are a looter!)
and #3, the more popular calibers will be the first ones taken, so odds are small you will be able to resupply during the duration of the emergency. (and even smaller you will be able to legally "resupply". Remember the rule of law will return and there is always the possibility you will have to answer for your actions.
Choose what suits your needs best, and supply yourself with enough to get through any reasonably forseeable tough times. Remember also that if the law is suspended (martial law) sales of all arms, ammo, and alcohol are legally suspended as well. So even if was physically available, you can't legally buy any.
Since you mentioned defense, as in defense against humans possibly, the SKS if the winner. The other guns mentioned aren't much suitable to that role.
This is another point. There is a significant difference between justifiable self defense with a rifle and the fantasy of holding off gangs of mutant biker ninja poodles...
An SKS, or something similar, designed for infantry COMBAT will certainly serve. However, even though not optimized for combat, any sporting rifle will provide accurate shots out to ranges where justifiable self defense becomes a questionable matter.