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.
Step 2: Pointy ammo
1. Box of nails, the sharper the better, I used 8D finish 2-1/2" they do not have large heads on them.
2. Ball of yarn
These are the easiest type of ammo to make. Simply cut small strips of yarn (or cut 1 long strip and fold it) and tape it to the back of the nail. Make sure that the tape has a good grip on both the yarn and the nail, so it wont fall off when fired. There needs to be enough yarn so that it will block all the air behind the nail when it is in the gun, but not enough so that it causes excess drag when it is fired. Just experiment until you get the perfect amount of yarn to use.
Step 3: Explody ammo
1. Drinking straws
2. Strike anywhere matches
3. Flash powder http://www.instructables.com/id/E65PVI1DFCEPUCHXG0/ I love tying my instructables together, its like a giant web of destruction
Cut the drinking straw to slightly shorter then 2 match lengths. Stick as many pieces of yarn into one end of the straw as possible. The yarn will not only act to to help accelerate the straw, but will also act as a plug preventing the flash powder and match from falling out, so make sure there is plenty of yarn. Insert one match into the other end of the straw, so that the head is facing the open end. Fill the straw with flash powder until the head is reached, but not submerged.
(Warning: Do this step roughly and youll be missing fingers, make sure to avoid excess jolts on the match heads. A good idea is to go outside and play with the strike anywhere matches, if you are unaware of their sensitivity, thats right play with matches to be safe. Try striking them on different surfaces until they ignite, notice the different amounts of force required to ignite heads with larger white tips, as you will see they are not overly sensitive. In fact they are almost not sensitive enough to make this work, but still better safe then sorry.)
Insert the other match, with the head facing in, so that the 2 match heads are in contact. Pour in slightly more flash powder to cover the 2 heads, if you want you can even fill up the rest of the straw with flash powder for a larger boom (Legal note: I almost forgot, dont use more then 50 miligrams its illegal. Even less then 50 is probably illegal depending on what state you are in. If 50 miligrams isnt enough to fill the straw, you can always mix 50 miligrams with gunpowder as filler, it should still have a nice report to it.)
With both matches in the straw, stuff around the matchstick with tissues or something so the powder does not leak when the straw is horozintal. Just dont stuff it so tight that it prevents the match stick from moving.
Step 4: Zappy ammo
1. Photoflash capacitor. The most important thing about it is that it is small enough to fit in the tube. I forget what type of disposable camera I harvested the one I am using from. So the only thing I can suggest is just to test out a range of capacitors, and when you find one that fits perfectly see if the maker sells them in bulk.
2. The tazer glove http://www.instructables.com/id/ESJUFG9VBJEPUCHSMG/ (the web of danger continues) would greatly simplify this, but if your too lazy to make one all you really need is a disposable camera with exposed charge leads.
3. 2 metal pins
4. Soldering iron and solder
Solder the 2 pins onto the leads from the capacitor. Put some electrical tape around the base to provent accidental bridging. Because the capacitor is already wide enough to block most of the air behind it, just put a small piece of electrical tape on the back, this not only traps extra air, but prevents you from being shocked if you accidentally breath in, although if you are dumb enough to breathe in you deserve to be shocked.
Now you may be asking yourself "isnt putting a high voltage capacitor inside a metal tube, and then inserting that tube in my mouth extremely dangerous?" The answer is yes. However to minimize the chance that the leads will be bridged when the capacitor is flying through the tube, place a piece of electrical tape over the needles and bend it down both sides, so it completely coveres all parts of the needle. Push the 2 points just slightly through the tip of the electrical tape. You may not even need to push them through depending on how hard you can blow. The safest thing to do would to be to have the tips of the pins covered, and then driven through the tape and into the target by the force of the impact.
Step 5: Firing
For pointy ammo: Not much to it, just raise the gun to your mouth, aim it at the target, and give a strong steady blow until the ammo leaves the barrel.
For the explody ammo: Needs an extremely powerful blow to get it to ignite the matches, also needs a pretty solid target. I am currently working on a system that uses those "party-snaps" to ignite a powder, they seem to have a closer sensitivity to what would be the ideal for this project. They are simply tiny rocks coated with silver fulminate, Ill update if I can get a working dart made using them.
For the zappy ammo: Charge the capacitor just before you insert it in the tube. You can charge it by touching the leads to the 2 fingers on a tazer glove, or the charging contacts from a disposable camera. It is a good idea to wear rubber gloves when handling the capacitor to prevent getting shocked. Insert the capacitor into the tube and give a good strong blow. When the it hits the target, as long as it is not an insulator, all the voltage will be dumped into it.