I too suffer with torn o rings if I try to swap a gas capsule too soon on my Zastava GP45.
The size of the seal is, I think, 20mm od, 16mm id which gives a 2mm seal thickness. I am not sure what the seal material is, (not original), but if it is in contact with liquid co2 for a long time, it swells by quite a lot. This was when I tore the seal, trying to get the piercing cap back on and catching it on the edge of the cylinder.
I have also drawn a blank searching for urethane o rings, does anyone have any more information?
Cheers, Mark.
Too many guns, or not enough time?