CO2 will swell and destroy the common, off-the shelf O-rings you find in most places like local hardware stores that sell O-rings for plumbing use/water use. IIRC, like he said, I think you need to use urethane O-rings instead to avoid that problem.
Safe and Happy Shooting!
Ed Krzynowek, The Airgun Tune-Meister
"We can rebuild the squirrel. Make him stronger, faster...We have the technology"---Skyler M.