You'll definitely want to have a trigger actuated ball valve to hunt with. The power difference is tremendous, you'll definitely need every bit of power possible to take out a coon with a nail dart. Steel is pretty easy to work with, check out the black iron pipe or galvanized steel pipe commonly used for gas lines. You can even use them with a sprinkler valve, if you use a orbit sprinkler valve, you can use the manual lever as a trigger. That is probably the easiest way to build a fairly powerful airgun.