Most CO2 guns use a disposable cylinder, a powerlet, that is purchased often pre-filled with 12 grams of pressurized 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. 

So, as long as the CO2 cylinder is not allowed to cool significantly (through firing), the pressure will not decrease by a significant degree. However, at higher temperatures (e.g. on a hot summer day), this approximation does not hold true.