With a pistol on CO2 you always have a constant FPS.
Co2 Airsoft Pistols have 1 advantage on for example GBB pistols and that is that they always have a much more constant Joule/FPS value than GBB.
CO2 is not influenced by temperature so the Joule/FPS value is always very consistent whether it is -10 or 35 degrees. GBB, on the other hand, is enormously influenced by temperature and is therefore the reason why there are various types of gases on the market.
Disadvantage of CO2 is that you always have to change the cartridges with either an Imbus key (often supplied with the magazine) or a cover attached to the bottom of the magazine.
With this you actually screw the CO2 cartridge into the magazine.