Green gas is simply propane fuel with a little bit of silicone oil mixed in with it. CO2 is highly compressed carbon dioxide gas. Green gas has a maximum pressure of about 150 PSI while CO2 has a maximum pressure of about 850 PSI. Generally, Green gas powered airsoft pistols will produce less velocity and be acceptable for use in close quarters combat situations. Some CO2 powered pistols shoot far too hard for use in close quarters.