Interesting that you should ask, I've been looking into this. "Airsoft Gas" is actually a liquid. The most common liquids are liquid propane and the refrigerant R22. At 70 degrees these are liquids only under sufficient pressures, about 110 ps for propane and 136 psi for R22. R22 eats the ozone layer and is suppose to be illegal, but it seems the King Arms green gas is still available as R22. After every shot more of the liquid boils away, so you get a large number of shots from a single fill since a little liquid turns into a lot of gas.
CO2 vapor pressure is 852 psi. If you regulated the CO2 down to 100 psi that the bolt was designed for it would be all gas. You get one shot and then have to refill.
And if the regulator fails or if you overfill it, the bolt will explode and you will be blinded, disfigured and/or die.
The pressure is a function of temp, as seen in these links:
http://www.glacierbay.com/ptchartpropane.asp
http://www.engineeringtoolbox.com/r22-p ... d_365.html
Hope that helps,
Phiz