There are three mechanisms of airsoft guns namely:
Electric powdered gun: They use battery power to boost the shooting mechanism of the gun. They are quite popular with people and are quite expensive. The biggest advantage is that the spare batteries are quite smaller in comparison to the gas canisters.
Gas powdered gun: The gas canisters are either attached to the gun by the hose or physically. The pressurized gas propels the airsoft gun BB's.
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In case you plan to buy the airsoft with small gas pistols, it is advisable to buy many gas canisters in one go. This will help you evade constant trips to the gas provider and help you save a lot of time and energy.
Spring-loaded guns: they are the least popular as they are neither automatic nor semi-automatic. After every firing, the spring needs to be re-cocked and this obviously hampers the rapid succession of the airsoft gun BB's.
Ask yourself what do you want to do with the gun? Hit the can, if yes then an inexpensive spring pistol will solve your purpose.
And in case you want to participate in airsoft games then, an electric or gas rifle is a must! The cost and power of airsoft guns are positively related. The higher the cost, the higher is the power of the gun.