http://www.airsoftretreat.com/gallery/data/624/medium/10.jpg
==== CA Note: ====
The CA piston o-ring has had a troubling past, being slightly too small to fully engage the cylinder on a highly upgraded spring. This problem could range from a small, 10 FPS drop, to even a 100 FPS drop. You can easily replace it with a larger one from your local hardware store.
=== IIg. The Spring Guide: ===