alrighty, let's see if I can word this. Compressed air is just air that is compressed (squished into a smaller space) Your normal air compressor can only compress air up to about 250-300PSI. A scuba tank compressor runs at 3000PSI.
CO2 has a very beneficial property in this case. When compressed to about 800 PSI (not exact, I know, but the theory stays valid) the gas goes into liquid state. The reason that CO2 tanks are filled by weight is to measure the amount of liquid. In a closed pressurized container, The CO2 will "evaporate" until the CO2 air pressure reaches the "800 or whatever"PSI, then the liquid will stop evaporating. The Gas will always stay at "800 or whatever"PSI no matter how much or little liquid is in the tank. Every time you pull the trigger on the paint ball gun the air/gas pressure drops a little and the CO2 liquid evaporates to compensate and bring the pressure back to "800 or whatever". This constant pressure allows for the discharge to be regulated. If you filled the tank with compressed air, once you went past the pressure that the gun was regulated to, the paintballs would drop in velocity until the speed of drool and then nothing. The same thing eventually happens with CO2 but not until all the liquid is evaporated and the remaining gas is exhausted. Since the liquid gas keeps the tank pressure constant, the liquid gas is like continuously refilling your tank. (until the liquid is gone of course)
I used to play a little paint ball, but mostly we used CO2 in laparoscopic surgery. Any trace gas that was left behind after the surgery was absorbed by the body and expelled through breathing just like the normal byproduct of breathing.
By the way, Some paintball guns are set up to use nitrogen, but they use it under the Scuba tank type pressure, not liquid. I can't explain the properties with nitrogen as my experience in that area was limited to cooling lasers. The people who used nitrogen paintball guns claimed they would shoot straighter and longer at the same velocity. In my experience, they fell to the storm of my paintball fury like everyone else!!!! MUHAHAHAHA
Hope this wasn't too boring, Steve