Correct o-ring materials?

[jirushi]

Hi,

What is the best o-ring material to use in CO2 airguns?

I have come across: nitril, silicone, urethane.

For instance, TR Robb sells QB78 seals that seem to be NBR 90 (nitril). For tube cap and/or valve body.
They seem to work well (as in, they don't leak) but someone told me that I should be using urethane.

I often come across white urethane seals in tube caps and valve bodies.
A few times, after discharging all CO2 and disassembling the gun thereafter, a white seals has made crackling noises.
I guess this meant that it had absorbed CO2 and wasn't made of the right material?
But perhaps white o-rings can be made of other material than urethane.

Then I have seen red and blue tube cap o-rings.
The red ones can be silicon. An American gentleman has told me that this is not compatible with CO2.
No idea what the blue one can be. Perhaps urethane with a colour.

I thought it would be good to do a post about this, as perhaps other tinkerers are confused by these o-ring materials too?
In pcp's it's easy: nitril. NBR 70 or 90.

Many thanks,
Louis