.41 Mag:
Look down a road. Look down high-tension wire right-of-ways. There's several places where you MIGHT have to shoot at long range.
Now, you also have to be careful about what people call "survivalism". A lot of people these days use that term because "militia" has been so demonized. There's people who want to survive, and those that have dreams of being a guerilla fighter.
The strict survivalist doesn't need any one particular weapon to survive. In fact there are a lot more things that are not weapon related that are needed more. The survivalist, by nature, will AVOID firefights, not go running headlong towards gunfire, like CowardKid thinks.
The wannabe guerilla will have his choices limited a lot more, as he will have to worry primarily about fighting other people, and survival is secondary.
But, either way, it's not a bad idea to have a long-range capable rifle. If nothing else, it will hit harder at the more common short ranges, and will have the capability to hit further out. If you "go kommando" and think the AR or AK are the be-al and end-all, then you are limiting yourself to about 250 yards. Yes, the AR (full size) can HIT further out, but not with much power..
Garand is also right about the whole "reload off the dead" thing. Unless YOU shot the person, and get the ammo RIGHT THEN, and take the ammo they had READY TO USE, you'd better avoid it. Why? It's an old trick to booby trap ammo with a caseload of Bullseye or C4. If you find ammo laying around, even if there is a body, it's too much risk to take. If you freshly killed someone (for whatever reason) I'd avoid the ammo from his ruck also, because it may have been "worked up" to lay down as a trap. However, it's unlikely that one would put trapped ammo on their web gear along with the ammo they intend to use.
So, in short, you won't be "harvesting" much ammo. If you DO get a fresh kill, guess what? you'll have his weapon also, and it won't matter if the ammo is the same as what your primary weapon uses or not. Ammo that is commonly found is also irrelevant. Stock up your own stash of it. After SHTF, you won't be able to walk into a gun shop or wally world and buy it. What wasn't bought or looted right away, will most likely have been confiscated to disarm the populace for easier control.
Basically, you "run what you brung", think accordingly.