SPRING

This is the most popular type of rifle, and generally a very simple one to operate. The rifle is cocked either by breaking the barrel or under-lever depending on the type of rifle. The rifle contains a piston and a large coiled spring. This procedure compresses the mainspring and moves the piston back, which connects into the trigger mechanism. When the trigger is pulled the mainspring is released, pushing the piston forward which pushes the air in front of the piston through a port and propels the pellet down the barrel. The movement of the spring and piston leads to recoil, which can affect the accuracy of the rifle. The lighter the rifle, usually the more recoil. To shoot a recoiling rifle accurately, techniques must be learned to compensate for the recoil of the rifle. Spring rifles are usually relatively cheap, simple and self-contained. They can be very accurate and powerful.

For
Self contained
No charging equipment required
Reliable
Relatively cheap to buy

Against
Recoil can make the rifle more difficult to shoot accurately, requiring more practice to become proficient.
Generally single shot only (there are a few multi-shots available)
The most accurate rifles tend to be heavy
Noisy to shoot compared to other rifles (PCP or CO2)

GAS RAM

Very similar to a spring rifle, but with a sealed gas unit or strut instead of the spring. They still have a piston, hence they do recoil, but they are the next step up from a spring rifle. They give less recoil and vibration, require less maintenance and can be left cocked for long periods without the worry of spring wear. One drawback however is the limited number of manufacture's who make these rifles, and therefore it can be difficult to find gun shops that stock these rifles.

For
Consistent power
No spring to wear out
No charging equipment required

Against
More expensive than spring rifles
Limited choice of rifles
Small amount of recoil

CO2

These use small CO2 bulbs (12 gram) or sometimes bigger 88gram CO2 bottles. The main advantages are they are relatively cheap to buy, recoilless and often lightweight. No recoil and bolt action cocking makes them popular with beginners, younger shooters and shooters who may struggle with the weight of a spring rifle. There are some disadvantages. You have to keep buying CO2 bulbs, they are limited in power and less consistent than other power sources. CO2 can vary in pressure depending on temperature, leading to inconsistent results as the weather changes. For this reason these rifles have a shorter consistent range, but are ideal for multi-shots.

For
Easy to use
Multi-shot models available
Relatively cheap
Usually lightweight 

Against
Average accuracy, particularly long range
Inconsistent pressure due to temperature change
Buying CO2 bulbs

Pre-Charged Pneumatic (PCP)

This is the one group of rifles that has advanced the most in the last 10 years. They use a reservoir built into the gun to hold a store of compressed air, usually about 2500-3200PSI. This reservoir can hold anything from 30-250 shots worth of air depending on the size of the cylinder. The compressed air cylinder is charged via a stirrup pump or divers bottle. These rifles have many advantages. They are recoilless so they are very easy to shoot accurately. They are very quiet when fitted with a silencer. There are many different models on the market to choose from, and they come in multi-shot or single shot. Some are available as both with the option to switch between multi-shot, and single shot. PCPs do however have some disadvantages such as cost, and the extra investment in charging equipment. Also the inconvenience of charging from an air cylinder or using a stirrup pump.

For
Very accurate
Full UK power (12lbs)
Large choice of brands and specifications - many multi-shots available
Very quiet with a silencer fitted
Large choice of add-ons such as silencers, bipods and custom parts