How temperature affects Airsoft gas

Airsoft gasses only work because when you open the container, the gas expands quickly through the mechanism of the gun, powering the shot.  The reason that the gas expands is because it is being stored as liquid under pressure.
 

The Science Lesson

There are three states of matter - solid, liquid and gas.  Airsoft gas exists in liquid and gas forms only so this is what we will be concentrating on.  When Airsoft gas is stored, it is stored in liquid form to save space.  This requires a high level of compression, which is provided at the point of manufacture and maintained until the gas is used.  The gas remains in a liquid state until it is released from its container, when it will quickly turn in to a gas.

The reason why temperature has such an effect on airsoft gas guns is because whenever a liquid turns to gas, heat is required to allow the change to take place.  This used heat is called the 'latent heat of vaporisation', and is taken from the general ambient heat present in the atmosphere directly surrounding the liquid gas container.  Without this heat present, the liquid would remain liquid when the valves were open.

This is also why these gas containers get very cold when fired excessively.  In these cases, so much heat has been required so often, that the gas container itself provides the heat energy for the reaction and then takes heat back from the atmosphere.  The mag will get very cold and can even allow frost to form all over it.  You will also notice that your supply can (the large gas can you use to fill up your airsoft gas guns) does not cool down dramatically while filling magazines.  This is because the majority of the liquid being transferred remains as liquid and never turns to gas - so no heat is required.  On some models, gas escapes during the filling process.  If enough of these were filled consecutively from the same supply can, the can would start to cool slowly, but the effect would be minimal.
 

So, how does this affect Airsoft?

So, heat is required for the change between liquid and gas.  The air around you ultimately provides this heat, so the colder the ambient temperature, the less heat there is available to change the state of the gas, so the state is changed more slowly, as heat can be found.  Of course, a slower change from liquid to gas, means a slower BB travelling down range, a slower reload on GBB replicas and generally less overall power.

If there is a lot of heat available to fuel the state change, then this change happens much faster.  This means that on warm days, the BB travels faster and the GBB reload mechanism is pushed much harder.
 

Choose the right gas for your GBB

Ok, so we now know why heat affects airsoft gasses, so what does this mean on a skirmish site, or in the back-garden?  Well, choose your gas well.

As heat plays such an important role in the operation of airsoft gas guns, you can either change the temperature to be ideal for your gun, or change the gas it is using.  Obviously it is easier to change the gas than the climate to suit your conditions on the day.

There are several different airsoft gasses available and they differ in their ability to change from liquid to gas.  Some gasses change easily (like r22 gas (green gas, winter gas)) and will provide excellent power, while others take longer to change (like 134a gas (summer gas)) and will operate the mechanism more slowly.  Of course, it is a good idea to use gas that is quite powerful so the gun becomes more accurate at long ranges, but this can have a MAJOR down-side - reliability.

As airsoft guns are made mainly from weak materials like plastic, they have a low structural strength.  For Gas Blow-Back pistols in particular, using a gas that is too powerful for the temperature is likely to cause severe or permanent damage to your gas gun.  Different models are able to support different gasses effectively, so it is worth checking which gas your pistol can support.  Don't just put r22 (Winter Gas) through your gun in the middle of summer to make it nice and powerful because it almost certainly will break.  There are very few GBBs that can do this reliably.

Talk to your retailer, ask on the forums, do your research and you should have a reliable, powerful GBB all year round.