Cylender volume vs barrel volume will also make a big difference. An example is in a vsr 10 the optimal barrel length is 500mm. When you start going to say a vsr10 gspec you will notice a huge rediuction in fps so say you have a full cylender ( no hole) and are running the "socom" style front set with the short inner barel you will have a lower fps the the same gun with a full sized front end with the long inner
http://arniesairsoft.co.uk/?filnavn=...andbarrels.htm
That link will help you figure the best barrel length for your cylender (longest one for the cylender is best)
Back to the vsr when using a gspec the inner barrel is ony 330 mm or shorter 313 depending.
Running that on the same setup as a 500mm one will lose up to 100fps. By shortening the compresion zone on the cylender the fps will go up once you shorten it enough the cylender and barrel will be matched thus no wasted air and the same or simaler velocity with the same spring on a shorter barel
Example
With the following compression zone lengths the 313mm length barrel chrono'd as such:
93mm ~400fps (no hop) ~6mm was taken off from 1st Project
79mm ~430fps (no hop) +30
70mm ~458fps (no hop) +28
64mm ~478fps (no hop) +20
64mm ~510fps (hop set for bb retention)
You can see how the fps goes up as the cylender is made smaller to compensate for the shorter barrel
Here is where the info came from
http://www.airsoftretreat.com/forums...?topic=88662.0