I have a KJW M700 that i converted to run on an HPA/CO2 system. Iv used the sites that Iggys and Hectic have both linked earlier to help me with my build. I have tapped both the small mag and the large mag but plugged the small mag as the large mag is much nicer (30 rounds instead of 10 rounds) and i dont mind the look of the long mag sticking out either.
My system will run on 12g CO2 in the way you see it in the photos but i can take off the 12g adapter and i can screw in a regular HPA or CO2 tank you find in paintball. I usually put the "rig" in my pocket as im going to be making a butt stock pouch for it soon.
on the 12g's i get about 100 to 120 shots running at about 100 PSI. I can adjust the PSI with an allen key as my regulator has a 0-200 psi output.
I used to use just propane but i had very poor consistency but now that i am using a regulated air source my shots go where i want them most of the time. So i am quite happy with my Air set up now but i still look for ways to make the air system smaller and more discrete.