http://www.speedairsoft.com/ProductI...ist&category=6
I built a piston for my BA aps2 clone and used a q-ring on it. I still need to try to verify, but I am probably loosing a good bit of fps from the drag of the q-ring on the cylinder body.
Holes in piston head face more to let air into the chamber from gearbox side than to aid in expanding o-ring