Propellants

Carbon Dioxide

Because CO2 becomes a liquid when compressed, it must expand to a gas in order to be used by the paintball gun. This expansion causes the tank to cool as heat is used to expand the liquid CO2 into gas. Eventually, under sustained fire, and especially in cold weather, the tank can become so cold that ice crystals form on it.

If the CO2 bottle does not have an anti-siphon tube fitted, or is shaken while firing, the liquid CO2 may enter the gun. The liquid CO2 then passes through the gun instead of the tank, evaporating and causing the gun to freeze. This results in large clouds of CO2 vapour ejected from the gun upon firing, caused by the liquid CO2 evaporating in/around the barrel. This is known as 'drawing liquid'. This can and will cause damage to internal seals and O-Rings, which will put the gun out of commission for some time while it warms back up.

High Pressure Air

When HPA is compressed, it remains a gas. When it expands, it also cools the tank, but at a far lower rate than liquid CO2 as it does not have to expand from liquid to gas. It is therefore seen as a superior source of propulsion. However, because these propellants are stored at up to 4500 lb/in² (31 MPa) while liquid CO2 is stored at 1200 lb/in² (8 MPa), tanks for nitrogen and HPA are more expensive at over £100. The tanks themselves can either be filled with pure N2 or compressed air, which is 79% N2. These air sources have traditionally been used primarily by people who play often and have tournament-grade markers; however, they are becoming more popular among casual players.

Comparison

Nitrogen is generally prefered over carbon dioxide for a few reasons. Nitrogen will not liquify and leak into the gun, while if the CO2 tank doesn't have an anti-siphon installed, liquid CO2 will leak into the gun, causing damage to O-rings. The solenoids on electronic markers are particularly sensistive to this, and thus many manufacturers will specify to use only nitrogen or HPA with their electronic markers.

Nitrogen generally has a more consistent shot than CO2. This is because when the playing area is warm, the CO2 will expand more rapidly from the liquid form, causing the gun to fire at a higher velocity. But when the temperature is lower, the expansion occurs more slowly, causing a decrease in the velocity of the shot. The effect of temperature on HPA or nitrogen, on the other hand, is negligible.

However, CO2 tanks are significantly cheaper than nitrogen tanks, which may cost between £100 and £300. The CO2 tanks cost slightly less to be filled, than the Nitrogen tanks at approximately £3-4.