These are all the parts you need! The whole launcher costs less than $50 and practically everything can be found on eBay. The muzzle break is the 2" union. You don't need this but I thought it looked cool as we had one laying around.
Then simply just assemble the parts so it looks like this. Use plenty of Teflon tape on the threads to prevent any air leaks and tighten securely with a spanner. Take the main body of the CO2 quick change so you don't scratch it (if you are bothered about it that is. I was anyway).
Cement the end cap or plug to one end of the PVC pipe. Prepare the surface with some sand paper, then use primer, leave to dry then the PVC cement. Drill a hole if anything a little smaller than the half inch elbow. Remember you can always file if it doesn't fit but it should be as snug as possible. Use Teflon tape on this too.
Then just screw it in. Not much more information needed here. Now you should be done.
Feel free to add as many unnecessary things as you like, maybe a laser sight for example. I went for an airsoft stock and strap. And if you used white PVC you could also spray paint it or add some camo tape.
To fire load a CO2 cartridge into the adapter with the smaller end at the top. When you screw on the shell a pin on the adapter punctures the CO2 and fills the pressure chamber. Aim and fire. As you can see on the left, snowballs don't work very well but it should launch a Nerf rocket around 300 feet.
Have to put a disclaimer in to finish, like many things these can be dangerous if used incorrectly. Obviously don't look down the barrel, don't shoot at someone directly, and don't put anything potentially dangerous down the barrel. I am not responsible for any injury that may be caused with this launcher. With that being said, be safe and have fun!
Parts List
All steel parts are galvanized - 2" Schedule 40 PVC pipe (class C UK) - 2" end cap or plug - 1/2" steel male to female threaded 90 elbow - 1/2" male steel nipple - 1/2" threaded ball valve - 6" of 1/2" steel pipe nipple - 1/2" steel pipe coupler - CO2 quick change adapter - PVC primer and cement - Teflon tape - Nerf vortex pocket rockets - 12 gram CO2 cartridges