CO2 on the only two o-rings in the gun will be fine. What happens usually with cheap o-rings and CO2 is the get saturated and swell up. The breach o-rings are only exposed for a very short time and not under pressure for long. Would be fine with most any o-ring imho…,but as always, the harder the better, get some 90 dura rings and have less to worry about:)
I’ve used cheap plumbing grade o-rings as breach seals in many of my CO2 guns with no ill effects, but of course would never use them in a gas filled tube.