Of all the things I have ever made, this is one of my favorites. Its an extremely simple and effective design and suprisingly powerful.
I include directions to make 3 different types of ammo for it. Regular pointy ammo, exploding ammo, and high voltage ammo. My favorite is probably the high voltage ammo, it packs quite a dangerous punch.
As always this project is dangerous, and I think it goes without saying "dont shoot at people, unless its self-defence, then certainly by all means go ahead and shoot at people."
Step 1: The gun
Just a simple 5/8" wide 4' long piece of copper or brass tubing. When you buy it make sure there are no kinks or bends in it or else it looses its accuracy.
You can vary the length of the tube to suite your needs / lung capacity. A longer tube will have a greater accuracy and will add speed to the projectile, but you will need the lung capacity to be able to forcefully blow a projectile from it, and a longer pipe is harder to carry around. I settled on 4' because it felt like a good length.
I also spray painted it black for looks.
If you want to be able to carry it over your shoulder you can attatch a shoulder strap. I used an old guitar strap and taped it to each end, but dont put it all the way to the end on the side you are going to blow from.