No. even though some spring guns have great range, co2 allows you to fire more then once before reloading.
Without knowing what your intended end use is, there is no way to answer the question.
It all depends on the strength of the spring airsoft rifle. If your refering to pistols (I will assume you are), I prefer a co2 or green-gas pistol over a springer anyday. In the big picture, springers are regarded as cheap and useless. Although the fps may be better than that of the gas, it's range (and the fact that you need to cock spring guns after every shot,) it is usually better to save up your cash and buy a good gas pistol. However, even I need to admit that some springers (i.e. quality snipers {runs you about 90-110 American dollars}) are way better than their co2 and green-gas counterparts.
Neither
No. The base of the guns are different and it is impossible to change it into a quality CO2.
Well no a Co2 cun justs uses compresed Co2 intead of a spring or a batery Co2 if fairly cheap and are easy to use
Most are electric, but many run on Co2 or manually cocking a spring.
it depends on their velocity. all three of them can have guns that hurt.
An air-soft gun that is powered by CO2 or green or red gas instead of a spring or a battery.
It depends on what type of gun you are using. Refer to the manual. I would say that Green gas would be better than CO2 because it keeps your gun in better condition, but use CO2 if your gun suggests that you should use HFC 134a.
No you do not need to put air in an airsoft gun. These guns are either spring powered where you must cock the gun before you shoot it, electric powered which are automatic and require batteries, or CO2 powered which requires a CO2 cartridge. This CO2 is not expensive and a pack of 40 CO2 cartridges costs only 20-25 dollars. the guns that require CO2 are not as common as the other forms of airsoft guns, although they are more powerful. In general, if you buy an airsoft gun, you do not need to put any air into it.
no
yes