There's still inside the magazine... The magazine basically creates a massive surface area for heat transfer...
Here's how I look at the HPA tank vs. CO2 thing:
Fills: Free (Generally around $5)
Weight on gun: 0
Weight on back: Unnoticeable
Attachment to gun: I have a sling...
shots: I do not know, but I'm going to go with less than 100, as that 13ci is listed as 150-300 shots, unregulated co2 is at around 850 PSI, and the cartridges are significantly smaller. (I know I am using the paintball measurements from tanks, but I will assume that the change is proportional)
Weight on gun: You've got the whole f---ing system on the gun, it will be heavier.
Weight on back: 0
Attachment to gun: Everything is on the gun...
HPA is more affordable in the long run, and if you drop your gun, maybe you have to replace a QD fitting, not an entire system...