'Nitro' is mostly a term used by paintballers for compressed air. Although nitrogen is used to fill paintball tanks, it is rare and compressed air goes in the same tanks, is cheaper and readily available.
CO2 is a cold liquid that vaporizes when depressurized and is widly used in paintball.
Each gas goes in a diferent kind of tank.
CO2 is easy to get and can be used without a tank regulator because it discharges at approxemently 800 to 850 psi. It can discharge into a marker as liquid as well and the pressure is variable so it can greatly effect performance and accuracy.
Compressed air is used in a tank that has a regulator on it to insure the correct output pressure. Because it is always in gas form, it is a much more stable propellent. The tanks for compressed air are much more expensive though and the compressors required are not on every street corner so it can be hard to find a fill station.
Almost all paintballers prefer air to CO2. And most high end paintball markers require air and will not run on CO2.
CO2, because it is cheap, easier to come by, and can be used unregulated, is still widely used in paintball. Most Spyder markers will run on CO2 or high pressure compressed air.
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