But I wonder how valid the "availability of ammo" concern really is. I mean, if you're lost out in the wilderness, the only ammo you have is what you brought along. It doesn't matter if it's 22lr or .25-20 Winchester. Even the most common ammo in the world isn't going to be laying around under bushes.
If you can't get ammo before
you leave home, you aren't going to have that wonderful 'survival' piece with you, at all. -It isn't about finding ammo while lost in the woods; it's about having the firearm with you, at all.
If you want to look at it as a survival weapon, you need to look at how external variables may influence its usefulness.