At least one gun of this rather broad category should be a part of the core set, primarily for those people who only intend to own a core arsenal. For those who wish to gather a complete basic arsenal, more than one subcaliber gun may supplement the package and be shared with the group, thus the second and possibly third subcaliber guns would be shared and used by several people. Subcaliber guns may be shared with other people for training purposes.
The most common subcaliber gun in the world is the .22 long rifle, like with the case of the shotgun, many governments have allowed their citizens to own these guns for sport hunting and recreation. The former Soviet Union had programs in high schools that ensured every youngster had the opportunity to learn marksmanship before serving their mandatory military service when they turned eighteen. The use of the .22 lr is authorized and encouraged by the International Olympic Committee and has thus been standardized throughout the world.
Shoppers can choose between a .22 rifle or handgun. There are several good .22 rifles on the market. I patriotically recommend any of the U.S. made rifles but lean toward those that take a detachable magazine. Most of them are of relatively good quality and are easy to maintain. .22 Rimfire guns (Short, Long, Long Rifle, or Magnum) foul up quickly and require more cleaning than other types of firearms but that is just part of the game. Extra cleaning and maintenance supplies are needed if the arsenal is to include .22 rimfire guns. .22 Rimfire guns are prone to malfunction and parts breakage which is tolerable in training guns but can be a liability in combat. Ruger .22s (rifles and handguns) seem less prone to have these problems than other brands of .22 firearms. The most commonly broken part of the .22 is the firing pin. Assure that you have at least one replacement fining pin in your maintenance kit for each .22.
The Ruger 10/22 seems to be the standard .22 rifle among survivalists but the Marlin Model 60 is more common among small game hunters. I think that the survivalist appeal of the 10/22 is because there are so many ways to fix it up with custom parts and accessories in spite of the fact that the manufacturer does not endorse the installation of any of these accessories. The gun can be made into a quasi-assault rifle with accessories from Butler Creek and Ram-Line like folding stocks and high capacity magazines. It can even be made into a quasi-sniper rifle with a heavy fluted barrel and big scope, or a fancy hunting rifle with a fine full length walnut stock. I personally would not sell the Ruger 10/22 short on the way they come from the factory. In my opinion, you really do not need to alter anything on the rifle to get pretty good performance out of one apart from adding a decent scope. Most gun collectors and survivalists consider the reasonably priced and good performing 10/22 to be a "must have".
The Marlin guns are particularly difficult to disassemble for detailed maintenance, but they use small parts (pins, springs and screws) that are commonly found in hardware stores. Marlin .22 rifles tend to be very cheap on the used market and spare parts are available at very low cost from the manufacturer. They are some of the best economical out of the box stand alone guns on the market. Mitchell Arms has imported .22 rifles from Yugoslavia that are made in the image of military assault rifles. This makes them a good low cost training guns for their bigger cousins, in addition to being respectable guns in their own rite.
Several of the .22 target pistols are nearly as accurate as .22 rifles but fit in a more compact package. I have never been able to figure out why, but decent .22 pistols cost more than decent .22 rifles. Two of the best choices are the Browning Buckmark and the Ruger MK II series. Both of these guns are very accurate and can be had in versions that are roughly the size and shape of service pistols so that they make good low cost practice guns. The importance of these guns lies in both in the inexpensive ammo they fire and the fact that they do not make nearly as much noise as the other guns. One disadvantage is that the controls can be slow to put into action and fairly large and heavy for the power you get out of them.
Some people use and promote the .22 for what they call "self defense", but more often than not, their concept is to murder somebody (maybe for a good reason but often not). The .22 is a commonly used weapon on the streets of the world and is quite lethal, but lethality and stopping power are two related but different things.
The pistol to the right might be deadly, but it is not a valid combat weapon. The pistol represents a good attempt by Wather in making a 22 pistol with true combat pistol features with a very rare double action first shot option. It makes a decent multi-purpose training pistol but lacks the accuracy needed for small game hunting. Thus it is marginal for the role of subcaliber survival gun and is at best a supplementary firearm.
Combat stopping power means that a projectile must strike with sufficient force to neutralize your attacker (the threshold of "sufficient" is a matter of heated debate among firearms experts), not simply cause him or her to die hours or days later. Some weapons like bean bag guns and Tazers have remarkable stopping power but little lethality. Of course a .22 can be used in a last ditch defensive effort, but it may not prove much better than a large knife at close range. .22s can be used to kill an armed enemy in some limited combat situations under 100 yds, but this involves remarkable tactics and skill (it has been done before).
A viable alternative to a .22 caliber firearm is a high quality airgun. Several manufacturers of airguns make rifles that are nearly as powerful as .22 pistols. The big advantage of these guns is that they take really cheap ammo and they tend to be highly accurate at shorter ranges. Airguns may also be your only choice if you live in a situation where the ownership of firearms must be kept secret and / or the firearms are cached in a location that is not readily accessible but you still need training weapons to be available. You can have thousands of rounds worth of airgun training for the cost of one box of regular handgun ammo. I do not emphasize the low grade kids' toys in the discount store but concentrate on the "adult" airguns like the Beeman, Benjamin, Walther or other European imports. The Chinese copies of the high grade European guns are also fairly good, as are the premium models put out by Crossman and Daisy, but only their premium models will fit the survivalist's need for a viable subcaliber gun.
Some of the lower grade airguns, especially the newer semi-auto models, are useful for pest control where more powerful weapons present a safety hazard. I have used a fast shooting inexpensive BB gun for protection against rattlesnakes while exploring empty city lots as a kid. I have also seen electricians and maintenance workers use them to shoot at rats and other animals while working in confined spaces under houses and around old semi-abandoned parts of commercial buildings. This could prove important if disease carrying animals become a problem in or around the retreat. Rats sometimes become a serious problem for recovery efforts after natural disasters. Birds can be a threat to some food supplies and fishing operations. It is usually safer to shoot at smaller pest animals with a lower powered airgun rather than a firearm.
No lethal, but hostile combat is another area where an airgun fills a potential need. I have seen pictures of store owners in Los Angeles using airguns to protect their stores during the 1992 riots. I suspect that this is because they felt that the airguns would cause enough hurt to deter looters without as much risk of causing serious injury or death as would be the case if they used regular firearms to shoot misbehaving citizens. Air guns are significantly more readily available than other alternative projectile weapons and are among the least costly. Airguns should NOT be used in training situations where trainees or trainers would be shooting at each other, use only paintball guns for that. There is always the off chance that somebody can be permanently injured with an airgun. Airguns require few accessories and little maintenance, the down side is that few airguns are repairable and must be discarded if they are damaged or worn out after the warranty period expires.