I did some research into this last year. It came out that there is an ideal ratio between cylinder and barrel, which for those that are interested is the cylinder should carry 1.6 to 1.9 times more air than the barrel. Like Embed said it doesn't really affect Spring Sniper Rifles for the reasons he indicated, its more of an AEG thing to increase efficiency in short stroked, fast and/or short barreled guns. This doesn't mean that the principal cannot be applied to a Spring Sniper Rifle in fact it might have positive effects, but as i don't have a spare cylinder for my VSR I am reluctant to experiment with it.
Here is a pic of what I did to my M4s cylinder when I cut the barrel to 4.5" long, sorry for the quality but camera phone tecnology ain't all its cracked up to be
. The section to the right of the line and holes is the end that the cylinder head goes in.
This is the modded gun
I still need to make a battery for it yet, and when its finished it will be my PDW.
Here is a pic of what I did to my M4s cylinder when I cut the barrel to 4.5" long, sorry for the quality but camera phone tecnology ain't all its cracked up to be

This is the modded gun

I still need to make a battery for it yet, and when its finished it will be my PDW.