Some notes on Airsoft guns -
There are 2 classes of guns sold under the airsoft name.
The cheaper ones are packaged with 0.12g BB and have a muzzle energy under 0.5j(0.37ft/lb). These are poorly made and are little more than toys.
The others are packaged with 0.2g/0.25g BB and are much better made and will have a muzzle energy of 0.8j or higher. it is also possible to get spare and upgraded internal and external parts.
As to power source. Airsoft guns are either gas or electric.
Most gas guns use canned gas sold specifically for the purpose(These are refridgrant gases, or propane, with added lubricants). Some new models use C02. Most of these have a gas reservoir in the gun's magazine behind the feed "tube" of BB. When fired the gas works the action to fire the shot and reload the next. They have to be manually cocked before the first shot is fired. If the real gun can fire full auto so can the airsoft version. The main drawback of gas guns is they do not work reliably below 12-15 degrees C.
Electric guns; called "AEG's" by the airsoft community, are cocked and fired by an electric motor powered by a rechargeable battery pack. With the exception of the electric pistols, MP7 and MAC10 all use Tamiya or deans connector to connect to the battery.
On magazines there are several types of AEG magazines -
Real Capacity(Realcap) - This hold the same number of BB as the real gun holds bullets
Standard Mag - These are only sold by Tokyo Marui - They hold approximately twice as many BB as a Reacap mag.
Mid Capacity(Midcap) - These hold approximately twice as many BB as a standard mag.
High Capacity(Hicap) - These use a hopper and clockwork mechanism and hold 10 or more times the BB as a realcap.
Tracer Unit And Tracer Ammo
A tracer unit has a UV LED, either in a silencer on the barrel or on the magazine feed tube. Tracer ammunition glows in the dark when fired through a tracer unit.
In the UK, since 1/10/2007 unless a registered member of an airsoft site or reenactment society you can only buy two tone coloured or clear plastic bodied airsoft guns.