Airsoft is a 'toy' for young adults on up. They shoot round plastic BBs, either single shot powered by a manually cocked spring, or semi-auto from a compressed gas cartridge. Some even offer burst or full-auto modes. They have a bright orange tip to designate them as a toy.
So why do I even mention them? Because there is a class of airsoft weapons that have extreme realism to real firearms (KWA). (kwaguns.com/professional.php) They are metal, and they fit standard holsters for their real counterparts. That means you can practice your carry, your draw, and your aiming till the cows come home, and you will neither break the bank, nor accidentally harm anyone, or even get the neighbors upset because of the noise. Other than training, they have no other purpose related to prepping. (goo.gl/ScXfYP)
The airguns of yesteryear (Red Ryder rifle etc.) shot small metal balls. Due to the fact that they have no ability to spin from rifling or the barrel, they are horribly inaccurate. So again, they have no purpose related to prepping.
And this is where we will spend most of our time, the pellet gun. Pellets, unlike BBs, are cylindrical and made of lead, so they hug the sides of the barrel, and the rifling puts a good spin on them for incredible accuracy.
There are a number of types of pellets; wadcutter (paper targets), round nose (hunting), pointed (hunting), and hollow point (of no use whatsoever) as some examples. There are also a number of calibers; .177 (same as BB), .20, .22, and .25. There are also some large bore for hunting large game like hogs, deer, coyote, bobcats etc., including .45 and .50 caliber. One last pellet type to mention that are worth having are felt cleaning pellets; just shoot your gun clean or use as non-lethal pest chasers (well, usually non-lethal).
I recommend the .22 caliber as it is easy and cheap to find in stores, and has a good weight and energy level. This caliber can take squirrels, rabbits, and even larger game if you can put in a well placed shot. For .22 caliber, a 14 grain pellet needs to move at least 615 feet per second to have enough energy. Rifles obviously give higher energy than pistols.
There are 4 main systems of power for pellet guns; CO2, Pumpers, PreCharged Pneumatics (PCPs), and Spring Piston.
Lets start with CO2 guns so we can get rid of them. The cons are the following: When you run out of cartridges after it has hit the fan, the gun becomes useless. Cartridges are expensive. As a cartridge is used up the gun's power and velocity drop off. In cold weather they often become un-fireable because the gas condenses, and in hot weather they can become un-fireable because the gas expands to such a high pressure that the hammer can't open the valve.
Now for pumpers. These old school guns have a compressed air chamber and a built in pump to fill it with compressed air. You can select your power level by how many strokes (within reason) you pump. These have two problems: The air chamber needs a system of seals and valves to allow the pumped air into the chamber, and a valve operated by the trigger to release the air; these seals and valves always fail over time. The second problem is that the air, as it is compressed, heats up (diesel effect), which expands the air and the pressure; but in the time between pumping and firing the air cools off and the pressure drops.
PreCharged Pneumatics, called PCPs, are similar to pumpers except they use an external air pressure system and MUCH higher pressures. Most people using these charge them from SCUBA tanks before heading out into the field. Many allow a bicycle pump to be used, which takes a lot of work. All of the big bore guns are PCPs. These are much better built than the old-school pumpers, but still suffer from the diesel effect, and eventually the seals and valve failing.
And now for the best choice, the Spring Piston. The original spring piston guns used a metal spring which is cocked by a single stroke of either the barrel (break barrel design), or a side lever or under lever. Today's spring piston use a nitrogen sealed piston (Nitro) like those that hold open the hatchback on cars. A single stroke compresses the piston. There are no seals and no valves to fail. Additionally, as the air itself is not compresses until the spring is released by pulling the trigger, the diesel effect works for you, not against you; as the air is compressed by the released spring, it heats up, expands, and gives an extra boost of power.
Spring piston guns must never be dry fired. Without the back pressure that builds up by the accelerating pellet, the piston would slam home and destroy the gun. Also, when the occasional oiling is done, you MUST use high flashpoint airgun oils. Due to the diesel effect, the air in the piston soars to hundreds of degrees, and low flash point oils will detonate, possibly killing the shooter. Even the correct oils will sometimes leave smoke rings. And use those felt pellets to clean the bore.