There is a thread in the forum asking whether the 7mm or 8mm would make a good caliber for a survival rifle. When you walk into a pawn shop and you see a bunch of a certain type of firearms, that is usually not a good sign. If people like a product, they tend to hang onto it.
22 Long Rifle
The 22 long rifle should not need an introduction. The ammunition can be fired in rifles and pistols, firearms are not expensive and they have a long life expectancy. With the low recoil of the 22, parts last long then a full sized rifle caliber.
The low cost of the 22 ammunition makes it attractive to survivalist who want to stockpile thousands or tens of thousands of rounds.
The 22long rifle is effective on small game and does not do a lot of damage to the meat. When you use a 12 gauge on a squirrel, you might have to pick out a bunch of BBs. If you take a squirrel with a 22 short, you have 1 bullet hole.
A couple of my favorite rifles in 22 long rifle include the Marlin Model 60 and the Ruger 10/22. My Ruger 10/22 was bought in January of 1986 and is still going strong.
223 Remington / 5.56mm
Standard service round of the US military. there are survivalist out there that plan on using the 223 as their main survival rifle after SHTF, but I personally would not want to use the 223/5.56mm on deer sized game.
If you want a rifle for hunting after SHTF, there are better options out there besides the 223.