Whats up Beebo, an anti siphon tank for CO2 is basically a CO2 tank with a tube in it that prevents liquid CO2 from entering the gun and wreaking havoc on your o-rings and internals. Nitro (compressed air) will always make your gun more consistent, but considering that your gun costs the same amount as a good air system, it is questionable whether or not air is a wise investment to a tippmann. The velocity adjuster basically does just that, changes the velocity of the paintballs exiting your gun. Usually, anything over 300 FPS (feet per sec) is dangerous, not to mention painful. You use that to adjust the velocity to the requirements set by all fields. Do not adjust the velocity by yourself unless you know exactly what it will do to the speed of your paintballs. Ask a guy in your local shop about it. Hope this helps.
