The idea is to make the bolt double as a trigger by attaching it with a cable to the plunger which works the semiauto valve. The valve plunger system would be like Dr. Nerf's:

The main air tank is not shown, I was thinking of using regulated CO2 but I don't know if there's a easy cost effective way to do this.
btw, does anyone know where I can get PVC part files for solid works, I'll try asking bolt loser?