This is nothing new. It's been done, and a co2 powered nerf gun is freaking dangerous at nerf wars. I have played with somebody who took a c02 powered bbgun, air rifle, or something and added a coupler to it. It hurt a lot.
if you are afraid to play against him because it might hurt then you would realy hate playing against me and my gun. also it does take some skill to take a gun that shoots a certain type ammo and mod it to shoot a totaly different type ammo, and i think he did a very decent job on it. at least it does not look like something that rose up from the junkyard.
by the way the games i plain , this is what we use for ammo and we shoot at 230fps.
What you can do to turn the velocity down more is either put a softer spring in or cut a little off the one that is in it. If you need it i can give you some links where you can get parts, Is that a PT-Junoir?