I have a digital caliper, and I measured everything! OD is about .790", ID about .533", wire dia. is .128", looks like 31 coils. Actually, I've come up with a theory about this, but it's specific to the CFX, because I think the cause is the rotary breech. None of the reasons you listed apply in this case, no deisleing, no oil in the compression chamber, and no heavy pellets.
I think the reason for the failures is piston slam due to a design flaw in the rotary breech. That rear o-ring is supposed to seal peak pressure, yet still be soft enough to allow rotation for loading. I think the rotary breech is the reason the Shadow is generally considered more powerful than the CFX, that rear o-ring is almost certainly by-passing some air. And I think it also means the front seal is getting even more abuse than we realized. Not only is there the shearing action of loading, the entire rotary breech has to be getting slammed forward with every shot as well. That's a lot more abuse than a Shadow's breech seal has to put up with!
So here's the challenge for you machinist types. How do you increase the sealing strength of the rotary rear o-ring, while still allowing it to rotate for loading? I beleive the current o-ring is a Buna or Viton, 70 or 75 durometer, 20 x 1 mm. Bob says he's worked on this problem before, harder rings were virtually impossible to install without damage, and were too hard to turn anyway. I have a couple of thoughts, one would be a longer rotary to allow an extra sealing ring, the other is a seperate press fit seal that goes behind the rotary and ahead of the piston. I'll be playing with this, and will post any interesting results, but I'd love to hear any other ideas or suggestions. Later.
Dave