CO2 Guns are rarely fully auto, Best your going to get is Semi, So Be Happy with Semi and just be trigger happy (fast)
It's the way its made, it cant just have a continuous flow of the gas, it wouldnt work, what makes it shoot hard is the burst
Also, If you DID have a gun on fully auto, you'ld run out of ammo too much, Believe me. i have a gun that holds 50 and itll run out in 10 seconds (Thus why I want one of those M16s or the AKs or the Typewriters with the 700 round clips) for my wars atleast, I want to actually collect guns soon enough. I mean a softair rifle is good for fun, but its not gonna stop an intruder lol