Well, been getting this question a bit lately, and someone (blood_on_blood) recommended someone make a thread about it to help people decide.
So... you're new to pump, and you've chosen a gun after reading my other thread
and can't decide if you should use CO2 or Compressed Air (CA from now on)? Read on and you can make a more informed decision.
Basics on the Gases
CO2, or Carbon Dioxide, is the most widely known compressed gas in use with paintball. It's been used since the infancy of the sport from the size of 12g all the way up past 20oz tanks. It is actually filled as a liquid and then expands in to a gas when used.
CA, or compressed air, is just that. A compressor compresses (shocker) air into a container, ranging from 3000 PSI (pounds per square inch) and up.
CA is not as effected by changes in temperature as CO2 is.
CO2 is readily available at most fields and hardware stores, and is cheap to fill. As has been stated, CO2 is actually a liquid that expands into a gas.
Because of this fact, it is possible to have liquid CO2 flow in to your marker if held so that the tank is higher then the marker. When this is done, the liquid, and not the gas, is shot, thus reducing efficiency.
CO2 is rather susceptible to changes in temperature. When it gets colder, the liquid doesn't change in to a gas as quickly as when it is hot outside. This is why many players use expansion chambers in colder weather. Expansion chambers are little chambers that give CO2 just that much more room to fully expand into a gas, therefor increasing efficiency. It also can cause a sharp increase in velocity, which is unsafe.
There is something you can add to CO2 tanks called an "anti-siphon valve". This is basically a straw put in to a tanks valve in a way so that when a tank is laying horizontally, only gas, not liquid, co2 can enter the valve, thereby reducing the chance of liquid co2 in your marker.
CO2 canisters come in many sizes, from small 12g capsules used in stock class, to large 20oz tanks, and more.
Efficiency with CO2 depends on the marker, setup, temperature, and other factors.
Compressed Air (CA)
Compressed air is the standard for most higher level markers as there is no liquid CO2 to freeze and harm the internals of markers. Again, most fields are able to fill compressed air.
Compressed air isn't effected nearly as much to temperature changes as CO2 is, and as such is more consistent in colder weather, which is why many people in northern states prefer to use them in the winter as opposed to CO2.
Compressed air tanks tend to cost more than CO2 tanks, as they include a first stage regulator to help control the flow of the high pressure gases, and have a pressure gauge on them as well.
Most CA tanks are refered to by a couple of numbers, like 47/3000 or 45/4500. These can also be written as 47/3 and 45/45 respectively.
What these numbers mean are the size and pressure the tanks are built to contain.
A 47/3000 is 47 cubic inches (capacity) and 3000 PSI (pounds per square inch.
A 45/4500 is 45 cupic inches with 4500 PSI.
Regulators are generally used with HPA (High Pressure Air) tanks to reduce the pressure that's coming from the tank in to a more manageable pressure for the marker.
So which is for me?
Now comes the fun part--- To decide which gas you should go for. This relies more on preference rather then anything.
Some markers are made for CO2, while others might suffer a bit from running CO2, so keep this in mind.
If you plan to run stock class, then you're stuck with 12g CO2 cartridges.
Keep in mind that CO2 is greatly effected by cold weather, so if you plan to play in the cold of winter, you might want to look at compressed air if you can.
This is a preliminary version, there WILL be revisions and corrections made. It's late and I'm tired.