More useful info:
"I've had lots of questions from people regarding gases to use in their airsoft GBB guns. There is a lot of misinformation on the Web related to this issue, so I thought it was time to present some hard science facts about airsoft gases.
The four gases most used for airsoft GBB guns are Green Gas, HFC-134a, Red Gas, and CO2. Green Gas is also known as Top Gas and is, in reality, nothing more than propane (with some perfume and silicone oil added). The main use for HFC-134a and Red Gas is in the refrigeration industry. Propane is also used as a refrigerant, but is primarily used as a fuel. In the refrigeration industry, propane is known as R-290 and Red Gas is known as HCFC-22.
The properties that make these gases good refrigerants is also what makes them good propellants for airsoft guns. At room temperature and atmospheric pressure they are all gases, but they can be liquified by putting them under pressure. When a gun's magazine is filled with one of these gases, what actually enters the magazine is a combination of liquid and gas (that's why you invert the can to fill the magazine). As long as there is still some liquid in the magazine, the pressure of gas above the liquid remains constant for a given temperature (this is called vapor pressure). As long as the temperature doesn't change and as long as there is some liquid in the magazine, the pressure of the gas propelling your BBs remains constant. Here are the vapor pressure vs. temperature data for all four gases.
It is easy to see from the data why guns with plastic slides need to use HFC-134a (stock TM Hi-Capa 5.1, Western Arms, etc.). The pressure of propane (Green or Top Gas) or Red Gas is simply too high for them to handle unless the magazine valve has been modified to limit the flow of gas. It's also easy to see why CO2 requires a special magazine and valve system. It's vapor pressure at room temperature is more than seven times larger than propane. Using CO2 in a magazine designed for Green Gas (propane) would blow the gun apart when it was fired.
If you are using an unmodified GBB pistol with a plastic slide, you should stick to using only HFC-134a. If you have a modified gun that includes a metal slide, enhanced recoil spring, and metal guide, then you can use Green Gas. From here on, we will stop using the term Green Gas and simply say propane. The long-term advantage of being able to use propane is cost. Propane is available through most discount stores and sporting goods stores for less that $3.00 for a 16 ounce can when sold as fuel for camp stoves (Coleman, Bernzomatic, etc.). Compare that to $15 for an 8 ounce can of Green Gas. Of course, you'll need an adapter to fill your magazines. The adapter can be purchased from Airsoft Innovations (http://www.airsoft-innovations.com/) for $25 and is shown below.
If you buy an adapter, be sure to buy some light-weight silicone oil (also from Airsoft Innovations). The oil is added to the propane to lubricate your gun and seals and also to reduce the flammability of the propane.
The adapter comes with instructions for adding the oil and filling your magazines. Another advantage to propane over commercial Green Gas is that the steel cylinders that propane fuel comes in are much safer to transport than the thin-walled cylinders that Green Gas is shipped in. The downside to using propane is that it doesn't contain the perfume that is usually added to Green Gas. Instead of smelling like petunias, your shooting area will smell a little like rotten eggs. (If your spouse complains, just remind him/her of the money you are saving using propane!)
A word about filling your magazines. If you've followed the discussion above, you'll realize that the pressure of gas in your magazine doesn't depend on the length of time you fill the magazine. As long as you get some liquid in the magazine, the pressure of the gas is independent of the amount of liquid present. The colder the magazine is when you fill it, the more liquid you will transfer to the magazine and the more shots you will get. However, if you get too much liquid in the magazine, then some of it will rush out when you fire the first few shots and spray you and your gun with liquid propellant. This wastes propellant and can damage parts of your gun (like the nozzle and piston seal) that weren't built to handle the pressure increase that results when the liquid vaporizes inside them. With a little practice, you'll find the right ""charging time"" for your magazines.
Edited by The Vainguard, 05 March 2010 - 09:42 PM.