Derek, I hope you will enjoy your new QB79!
In my experience, Buna works well for the barrel O rings (where CO2 exposure is intermittent and brief). We've found that EPDM and Urethane rings are superior for the valve O ring, however.
The factory has used a variety of O rings styles for the valve and tank block. The material for both O rings has normally been urethane in recent years.
But I have found that the factory O rings around the tank block tend to split apart at their join after some years and BUNA can work well there as the material is more flexible. Factory O rings in this position can be green, purple or cream-colored - the cream Urethane rings are generally seen on more recent examples.
I'm sure others will also chime-in with their experience, too
Archer Airguns Inc.
Online stores http://www.archerairguns.com