Watch this build and test of a high velocity vortex cannon

A low velocity vortex cannon isn't too hard to make, but a vortex cannon that's this high velocity takes a lot of trial and error.

Via his Instructables on how to make one:

If you've seen a vortex cannon in science class you're probably expecting a garbage can with a hole cut in the bottom and a diaphragm on the back. While you can make low velocity vortex rings this way, to make a really high power slug of air a cone shaped barrel is the way to go. Making such a barrel can be a difficult task. In previous iterations of this cannon I used fiberglass to form a cone over a paper mold; a messy process that is likely to have you breathing toxic resin fumes and spending quite a bit of money on materials.

Building a High Velocity Vortex Cannon (YouTube / NightHawkInLight)