There are a couple of types of air pressure systems used to fire paintballs. Make sure you get the system that suits your needs. Carbon dioxide is the most common system and the size of the tank determines how many balls you'll be able to shoot. Depending on the temperature, carbon dioxide can be either a liquid, gas or both. This often causes fluctuations in speed and shooting accuracy. This system is generally more safe and reliable for recreational players and most playing fields have stations available to refill tanks. Nitrogen and high-pressure air systems always work as a gas, so your gun doesn't suffer changing velocities.