I often suggest a common caliber hunting rifle, 30-30, .308, 30-06, etc. If the ZA turns into the clusterfuck that Hollywood portrays it to be, then Military grade equipment and resources are going to get used up quickly. Commonly used calibers are more likely to be hoarded and stored than military calibers. 9mm tends to straddle both but I would still stick to more common "Civilian" calibers. In my personal "armory" I have a .357 Single Action 6 shooter and a 30-30 Lever action rifle. I like both of these because the ammo is pretty common and the actions are simple and easier to clean/maintain. It has always been my opinion that ease of use and ease of maintenance is far more beneficial in a low resource scenario than bullet size...however, I understand that some of the lower calibers (.22 for instance) are generally looked down upon. I agree heartily with the person who stated that it has a lot to do with your geographic location as well. Almost everyone in my particular area has rifles and handguns in the calibers I mentioned above. However, a .22 could come in very handy as well for small game hunting. In terms of sustainability, small game hunting is generally better than large game hunting because specific geographic areas support more small game than large game. Such things as group size and nutritional needs would also become a factor but if you are in a small group (10 or less including small children/infants) you will likely never cease to find small game. .22 Caliber rifles are excellent for small game hunting. If you yourself don't carry one, perhaps someone else in your group who is a "non-combatant" (the old or infirm or even 10-12 year old children) could carry such a firearm if it is considered less useful in a combat scenario. However, one last thing to add, it should also be considered that we tend to base many of these ideas on human to human combat scenarios as opposed to human to zombie combat scenarios. This enemy is much different than the guy down the street that wants your plasma TV or the enemy combatants in some foriegn country. We may find that we need much larger calibers than common hunting rounds or that much smaller calibers will do. Don't be afraid to have a few different calibers (of guns) in your armory just in case...just my honest opinion!
I agree that we have no idea what caliber is going to work well, and what calibers will not. From my medical background I have a theory of how zombies, will be if they ever occur. I don't believe they will be dead rotting flesh, I think its more the case that they will be in a rabid state. Possible curable, possibly not curable. Everyone likes to think of zombies in the states we've all grown use to. Its fun, and entertaining to think of them that way. Chances are though, they will be living. They may not be as coordinated, running maybe out of the picture. If this is the case I still believe a larger caliber will be a better choice.
I like most hunting calibers better than 5.56 nato anyways, but Im not sure I believe that their going to more available. I think that is going to rely highly on your geographical location. I live in Florida, and while there is a lot of hunting going on here, its predominately hog hunting, and some deer hunting. the most common rounds used for that are 30-30, and 30-06. Sometimes depending on the stores those are hard to come by. I have several different hunting rifles, and acouple of rifles I use for long distance comps. So I keep other rounds in my armory, and I reload several other rounds. When I stated in my previous post that I would carry one of my AR's with my 5.56, and 6.8mm upper receivers. As well as a pistol with both 9mm, and .40 caliber barrels, and springs I was pretty much thinking on the lines of what Im going to carry if Im nomading it. Eventhough, those calibers are more commonly used, their also much more commonly produced. I still believe there is a good chance of finding them if your moving over hunting rounds. I'll give you an example of my reasoning. If your traveling either trying to get to a area you want to strong hold, or just traveling because you aren't safe staying stationary. You should always find a telephone book in the area you'r in. For instance I live in Melbourne FL. Not very far from my home there is a small local reloading business called Space Coast Bullet. I know some guys who work there. Now I know from talking to them, and shopping for reloading supplies there that they have a large amount of .40 caliber rounds. I know they also have a really large supply of 5.56, and .223 rounds. Their numbers in those particular rounds will run in the 30,000-50,000 round range. Their other rounds such as 30-30, and .357 and rounds the likes they have considerably less in...around the 10,000-20,000 range. They sometimes will set on those rounds for awhile waiting to sell. The reasoning for the .40, and 5.56 in particular is because our law enforcement agencies around here all recently switched to .40S&W. Back in the 80's it bacame standard practice to issue carbines to patrol officers aswell. I know that as the price of copper has gone up, these reload companys have opened up all over the US. Might be a good idea to check out local phone books to see if there is a place in a town you run into. Stake it out for a while and take a look when your confident its safe. My local loader is off the beaten path so far that most people will not be able to find him, and the only people who know its there are local shooters like myself. Its a safe bet, it could still have ammo. At the very least their going to have powder, and reloading supplies. Its extra weight, but if your not on foot its definitely worth taking with you. It maybe good trade material later, or it could come in handy if you find a damn gun for it.