I've built dozens of launchers and the most important thing to keep in mind is safety. If you are going to build an air canister to store the air to power your shot here are some things to consider....
~ Attach a gauge to the canister. (Schd-40 can easily be tapped & threaded) It's important to know just how much psi is in there.
~ Attach a burst valve, I personally use a 100 psi one. It ensures that my canister will vent if filled over 100 psi.... that is well under the rating at which my Sched-40 canister will structurally fail.
~ Use a 1" sprinkler valve instead of a ball valve. The sprinkler valve 'dumps' instantly, several times faster than you can turn a ball valve handle, and that means more fps per psi. This type set-up nets you around 200+ fps @ 85psi with a nerf rocket.
If you think safety is secondary when using these just remember where you're putting them right before you shoot.... right next to your head!