Propellant system

The tank holds a compressed gas used to propel the paintballs through the marker barrel. The tank is usually filled with CO2 or compressed air. High Pressure Air (HPA) is sometimes referred to as "nitrogen", as air is 78% nitrogen, or because these systems can be filled with industrial nitrogen. Due to the instabilities of CO2, HPA tanks are desired for consistent velocity. More exotic means of paintball propulsion include the combustion of small quantities of propane or electromechanically operated spring-plunger combinations similar to that used in an airsoft gun.

CO2
Carbon dioxide, or CO2, is a common propellant used in paintball, especially in cheaper guns. It typically comes in the form of a 12 gram powerlet, mainly used in stock paintball and in paintball pistols, or a tank. The capacity of a CO2 tank is measured in ounces of liquid it can store. A CO2 tank is usually pressurized at around 1800 PSI. At this pressure, CO2 tends to form a liquid. When it is released, only a portion of the CO2 is stored as gas, and the rest (liquid CO2 in the tank) must first vaporize into a gas before it can be used. This process leads to some commonly-known problems such as inconsistent velocity. It especially has problems in cold weather, which slows vaporization and increases the chance for liquid CO2 to be drawn into the marker. Liquid CO2 injected into the marker is very cold, and can damage some of the internal mechanisms. Anti-siphon tanks have a tube installed inside the cylinder which is bent to prevent liquid CO2 from being drawn in. They must be screwed in so that the tube faces upwards to function properly.

High Pressure Air
High Pressure Air (HPA) or compressed air or nitrogen is stored in the tank as a gas, bypassing the evaporating issue. Air is stored in the tank at a very high pressure, typically 3000-5000 psi, and output is controlled with an attached regulator, this regulates the pressure from 250 psi to around 850 psi (the output pressure of CO2) depending on the type of tank. This results in a much more consistent velocity than with CO2. HPA tanks have two measurements: PSI and in3.

HPA tanks are more expensive, since they must be rated for a much higher pressure, and are typically made from more expensive materials. Tanks can be purchased that are steel or aluminum, or wrapped carbon fiber bottles. Carbon fiber bottles are much more expensive ($150+ USD) because of the price of materials and manufacturing.

Gas regulation
Marker systems have a variety of regulator configurations, ranging from completely unregulated, to some high end systems using four regulators, some with multiple stages.

The regulator system affects both the accuracy and at what velocity the marker operates(in conjunction with valve dwell/hammer pressure). CO2 regulators have an additional job of making sure no liquid CO2 enters the marker and expands, which causes a dangerous velocity spike. As such, regulators for use with CO2 often sacrifice throughput and accuracy to ensure the marker operates in a safe manner. HPA-only regulators tend to have an extremely high throughput and have design considerations to ensure uniform pressure between shots for marker accuracy at high rates of fire. Tournament markers usually are equipped with four regulators, each with a specific job. The tank regulator first moves the 3000psi to 4500 psi HPA (in CO2, this is not present) down to a more predictable pressure, usually between 600-800 psi. A second regulator is then employed, which regulates this pressure to one close to the firing pressure, and is a matter of personal preference between players. A pressure closer to the final allows for greater consistency, but a larger difference reduces shoot down. The air is then supplied to a regulator on the marker body, where the final output pressure is decided. This can be 800 psi for entirely unregulated CO2 markers, to 150-200psi for extremely low pressure markers. Some markers have great success in a low-pressure conversion, while others do not. After the firing pressure is decided, tournament-oriented markers will use one more regulator to supply gas to a separate pneumatic system for any other functions, such as bolt movement. This is an extremely low volume, extremely low pressure regulator, usually under 100 psi.