Matching cylinder capacity to Barrel length.

[North West Outpost]

I have just posted this in another thread but it was wandering off topic, so I thought it may be worth having on it's own.

If you have browsed any Airsoft shop you will find they sell cylinders with holes in called ports.

If you look at the different types offered you will see that there are ports at differing distances down the cylinder. The ports are located such that they offer you the chance to use a cylinder that matches your air required by the barrel to the capacity delivered from the cylinder.

So a longer barrel may not need a port where an MP5 has a port a long way down the cylinder.

It isn't an exact science as the manufacurers differ as to where the ports are and offer a range of barrel lengths per cylinder.

On top of all that, each set up is different so that will impact the required air. The higher the fps the higher the ratio of air in the cylinder to barrel required. But for our usual fps's you don't really need to worry.

If you get the balance fairly close your gun will be much more efficient, the piston still travels past the port but on it's forward stroke the unwanted volume of air passes out of the port rather than pushing the bb out of the barrel.

If you have an unported cylinder in a short barreled AEG as above the piston will still travel it's full distance but instead of the unwanted air exiting via a port, it is now used to prematurely push the bb out of the barrel. Requiring a higher rated spring to offer the same fps. Or just giving a lower fps.

In the above case it is often ok to run a gun like this but it will make them more noisy as the bb has left the barrel way befor the piston head slaps the cylinder head. It is like dry firing, adding additional wear and tear.

At the other end of the scale, you will encounter problems if you use a long barrel and a cylinder that offers to little air. In this case the piston head will have slapped the cylinder head befor the bb has left the barrel. So fps and range will be hopeless. If used on auto further impacts the system. As you can imagine air pressure suddenly stops with a bb in the barrel it creates what is commonly known as suck back. To compound that on auto you have also loaded another bb into the hop rubber to seal the barrel whilst still having the previous one too get rid of.

There are other determining factors such as using sorbo or washers to correct AOE, they both slightly pre cock the piston which will take away cylinder capacity, as will short stroking. So if you use both methods in an extreme rof gun you deffinatly need to consider these things. Short stroking per tooth will decrease the cylinder capacity by approximatly 5 -6% (3mm ish) so 4mm of sorbo will be a little more say 7%. These are rough figures but it gives you an idea. Based on a 16 tooth set up not 19.

There are a few other exceptions not covered here.

Silent sets, where the heads are both conical and are supposed to offer better air flow due to this.

Bore up kits, slightly larger internal diameter offering slightly larger capacity per mm travel of piston. Not usually required really only in extreme cases.

Air brake piston heads. Most are just generic made and don't match the air brake diameter to the cylinder head nozzle diameter so are gimicy at best and not reccomended in extreme rof guns for the obvious reason that they slow the piston head down.

I hope that helps.