".308 being more available/common that 30-06 is a myth."
I agree. 30-06 is generally more available in my neck of the woods. But, 308 is readily available, too.
But, to those who wish to prepare for any possibility, more power to you. For me, it is a tremendous waste of money for that to be the goal. I have an FAL, AK, and Mini-14. I also have an SVT-40 and M1 Garand. That means I am prepared for invasions of every kind, right? Nah, just interested in those designs. Semantics? Perhaps, but I am a bit more comfortable about MY neighbors and fellow citizens in south Mississippi. We tend to get along pretty well. If civilization collapsed, it would not really collapse locally and we would probably continue okay. See, in all the places I lived, the Great Depression affected the populace but not to the John Steinbeck extent. No, folks merely continued to farm, hunt, fish, make a living. Money was short, but then it had always been short so nothing changed.
Today, there are a larger number of helpless folks who would have trouble should bad things happen. But, the salt of the earth still remains around here and I doubt too many bad things would happen. At least, after Katrina, nothing bad happened. The police were busy, but not at looting. Security was heightened, but nothing went wrong. We got in line at the artesian well for our daily share of water (no running water). It tasted a bit rusty, but it provided us our need. That well had been capped and ran directly into the river for decades. After Katrina, the city uncapped it and let it run free over the ground. That was our solution. No water? Okay, let's uncap that well over there. No power? Okay, let's clean things up so we can get it restored. It took weeks to get power here, much longer in the more rural areas. But folks managed to keep their clothes on and did not paint their faces ala Lord of the Flies.
So, having experienced SHTF, I am very comfortable about how to endure in my neck of the woods. Of course, I found out about New Orleans only after several weeks of taking care of business in my own home. Guess southerners are just more civilized. (No, New Orleans is not in any southern tradition anymore).
Having lived through Fredrick in 1979 and several smaller hurricanes in the intervening years, and having had family go through Camille in 1969, I have always been hurricane prepared. Being hurricane prepared means you ARE prepared for SHTF.
So, any rifle will do. You will not run out of ammo if you have several boxes at home. If you really are worried about SHTF, then spend more time on pioneering skills. That would really be more valuable than which rifle will do best. Any rifle in a common caliber and of a rugged design will fit the bill. Minutia of which ammo is carried by the military is way, way down on the list. Think about that after you have your own emergency shelter in the mountains somewhere, know about riding horses, how to build a real shelter, know how to determine potable water, how to tie proper knots, build a proper fire even in rain and without matches, etc. Those are actually more valuable skills to focus on.
Ash