There was a thread with pics of cylinders with different location of air holes. I will dig it up if I can.
<--this one is the one I remember bookmarking
: The Cylinder is a metal tube which the piston/piston head passes through when firing. It's the channel for the air to reach the barrel. There are two standard types for AEGs, Ported and Non-Ported. Ported cylinders allow air to be collected from inside the mechbox as opposed to collecting it from the barrel when the piston is returning to it's starting position. This is needed for long barreled weapons with a high rate of fire to ensure that the trajectory of the bb isn't effected by air being sucked back into the barrel before the bb has been expelled. If you're expirencing a drop in power and accuracy after upgrades, you may wish to consider a ported cylinder."
Interesting read http://airsofthawaii.com/forums/view...c8f5d181ab3194