Wow, there's alot of CO2 myths flying around this thread.
I'm no expert, but I've done a bit of research on running CO2 since HPA isn't a viable option for me. Anyway, this is how I see it:
Mags having trouble with liquid, and not running well in the cold on CO2 are 2 different issues.
Get liquid in your mag and the orings will swell, causing the action to jam. Anti siphon your tank, and make sure the orientation is proper and you won't get liquid in the gun whether its winter or summer.
The reason mags don't run well on CO2 in cold whether is because below 50-60 CO2 only has around 600psi of pressure. Mags need 800+ to run well.