CO2

Most CO2 guns use a disposable cylinder, a powerlet, that is purchased pre-filled with 12 grams of liquefied carbon dioxide, although some, usually more expensive models, use larger refillable CO2 reservoirs like those typically used with paintball markers.

Carbon dioxide-powered guns have two significant advantages over pre-charged pneumatic air guns: (1.) A simpler system for compact storage of energy—a small volume of liquid converts to a large volume of pressurized gas. (2.) No pressure regulator. Within a temperature range tolerable to humans there is little need to regulate the inherently suitable pressure for low-to-moderate-power air guns. The vapor pressure is dependent only on temperature, not tank size, as long as some liquid CO2 remains in the reservoir.

These two advantages allow CO2 guns to be constructed more simply than guns using a pressurized air reservoir. Some CO2-powered guns have detachable or fixed reservoirs that are loaded with pressurized gas from a larger cylinder. Most CO2 powered guns use the standard 12 gram Powerlet disposable cylinder invented by Crosman. Recently, the same company introduced a new 88 gram disposable AirSource cylinder that is used in some of their guns.

On the other hand, liquefied CO2 must be purchased, which introduces an element of cost that does not factor with a PCP gun/hand pump combination using "free" air, or is at least considerably lower when refilling from a diver's tank.

Furthermore, the pressure of gaseous CO2 at ordinary ambient temperatures is only around 850–1000 psi (6 to 7 MPa), which is only a third of the safe working pressure of a typical full PCP reservoir (20 MPa or 2900 psi or more). The effect of this is that generally speaking CO2 guns are lower powered and less efficient than PCP guns, which is why CO2 guns are usually pistols or semi-target type rifles, with few guns (none of commercial note) reaching even the 12 ft (3.7 m)·lbf (16.2 joules) licence-free energy limit for air rifles imposed in the UK.

CO2 guns, like compressed air guns, offer power for repeated shots in a compact package without the need for complex cocking or filling mechanisms. The ability to store power for repeated shots also means that repeating arms are possible. There are many replica revolvers and semi-automatic pistols on the market that use CO2 power. These guns are popular for training, as the guns and ammunition are inexpensive, safe to use, and no specialized facilities are needed for safety. In addition, they can be purchased and owned in areas where firearms possession is either strictly controlled, or banned outright.

Most CO2 powered guns are relatively inexpensive, although there are still a few precision target guns on the market that use CO2.

The CO2 system has been used in experimental non-lethal law enforcement weapons, where high power delivery systems launch rubber batons or bean bags out of a gas-powered launcher, much like a non-lethal shotgun system (but at lower velocities, thus being safer).

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