In the world of airsoft guns first comes spring powered and then we have gas powered airsoft guns. Spring airsoft guns are great fun and totally functional but gas models take the airsoft hobby a step up. The annoying aspect of spring airsoft is having to actually cock the gun every time you fire it – whether it's a pistol or not. Gas airsoft guns contain a gas charge (since the name) so your single shot days are over and you can enjoy a more automatic firing experience. These guns work in the same way as the spring ones in that piston compresses and drives air through the gun barrel to fire the bb but because there's a "constant" gas charge in the gun you can fire several dozen rounds without having to worry about cocking it again.
You'll notice that most airsoft gas guns are hanguns or small machine guns – the Uzi being a perfect example. There are a few gas powered rifles but these only ever seem to be sniper models and never assault rifle replicas. A full sized airsoft assault rifle could deplete an entire gas charge or even a tin of compressed gas in just a few moments. That same amount of gas would provide an airsoft handgun with sufficient power to fire off several clips of bbs and leave some gas to spare.
Your airsoft gas handgun or machinegun is recharged through a small nozzle at the base of the gun – usually right underneath the handgrip or near where the ammo clip is inserted. Simply press a can of gas into the nozzle and fill the gun until it's fully charged. For the first few bursts the gun will be operating at full power and then it will slowly die off until it's a feeble popping noise and it'll need to be recharged again.
Running an airsoft gas gun is not free but it does not cost the earth either. The running costs are, however, something you need to take into account before buying one of these guns. A 1000ml tin of gas will cost about $ 8 or alternately you can buy a gas pump that you carry around with you. This is pumped to a certain pressure and you can then recharge the gun from this. The only downside to these is that you have to carry the pump around with you as it's permanently attached to the gun itself.
Because airsoft gas guns contain a sealed gas chamber that they need to be handled with extra care. Personally I've destroyed one Desert Eagle and a Beretta by dropping them on hard ground and cracking the case of the gun. These proved impossible to repair and the guns had to be thrown out. Treat any gas powered airsoft weapon with extra care – you'll thank yourself in the long run.