The tank holds a compressed gas used to accelerate 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. Due to the instabilities of CO2, HPA tanks are desired for consistent velocity.
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 3000 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 450 psi to around 800 psi 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, but the most widely used are wrapped carbon fiber bottles, due to their light weight.
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 guns 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.
Barrels
A barrel serves to direct the paintball and control the release of the air pocket behind it. Several different bore sizes are available, to best fit different sizes of paintball, and there are many different lengths and styles. Most modern paintball markers have barrels that screw into the front receiver, as opposed to some older types which slide a barrel on and screw it in place. Barrel threading must be matched to that of the marker. Common threads are Angel, Autococker, Impulse/Ion, Shocker, Spyder, A-5, and Custom 98.

Barrels are manufactured in three basic configurations: one piece, two piece, and three piece. It is more common to refer to a barrel with interchangeable bores, be it two or three piece, as a barrel system, rather than a two-piece or three-piece barrel. This avoids confusion, as many two-piece barrel systems do not use an interchangeable bore system.

One piece barrels are machined from a single piece of material, usually aluminum. Paintballs can range from .669 to .695 caliber, and barrels are honed to match these diameters. Many one piece barrels have a stepped bore that increases from their rated bore size to around .70 caliber after eight inches.

Two piece barrels consist of a front and back. The back attaches to the marker and is machined with a pre-specified bore between .682 and .695 caliber. The front makes up the rest of the length of the barrel and contains the porting. Fronts usually have the same bore as the largest back the manufacturer offers.

Three-piece barrels, instead of having multiple backs each with a different bore, have only a single back. A series of inserts, or sleeves, with differing bores are inserted into the back. The front is then screwed on to keep the sleeve in place. Sleeves are generally offered in either aluminum or stainless steel. This type offers the most flexibility in that the user needs only one set of sleeves and a back for each marker they own. They can also select front sections to make the barrel length they prefer. This type also generally offers the widest selection of barrel diameters, usually .680, .681, .682, .683, and so on up to .696 caliber.
Length
The length of barrels can range between 3 and 20 inches, though some custom barrels exist which may reach up to 48 inches. Recent tests have suggested that neither accuracy nor efficiency can be gained in barrels beyond 8 inches long, as a paintball needs an unported 6 to 8 inches to stabilize and achieve maximum gas efficiency. Longer barrels generally make less noise than shorter barrels by allowing excess gas to escape more slowly from porting along the sides. Longer barrels can allow players to "sight in" faster than they would with shorter barrels and thus give the perception that longer barrels are more accurate, however a good set of sights is more accurate. The barrel choice commonly used by professional players is 12-16", as it keeps the balls shooting straight, but the marker is still light and manuverable. Many professionals will also choose the longer barrels as it allows them to push aside the large inflatable bunkers commonly used in professional paintball tournaments while still staying behind cover.

Most barrels are ported (or vented), which means that holes are drilled into the front of the barrel allowing the propellant to dissipate slowly, decreasing the sound signature of the marker. There has been a considerable amount of marketing hype surrounding barrel porting, but there appears to be no basis for claims of the benefits of porting (such as that it decreases "turbulence", increasing accuracy), other than the decrease in sound signature.

It should be noted that porting in the first eight inches of the barrel length decreases a marker's gas efficiency. For example, if a 16 inch barrel has large porting that starts six inches past the threads then it has an effective barrel length of only six inches. At that point, the ball has to travel the other 10 inches on its own momentum. The friction within the barrel must be overcome with a larger burst of gas, decreasing efficiency. Porting in a barrel before the paintball has completely stabilized can also dramatically increase noise, as the gas still has a significant amount of pressure built up.
To be Continued