You will need:
- A thick plastic canister. (the thicker the better)
- A tube. (Aluminum works the best but plastic can also do the job)
- A piezoelectric igniter.
- 2x metallic screws.
- Plastic coated wire.
- Epoxy (or similar).
- Sandpaper (optional but recommendable)
Important: The canister will have to deal with a strong pressure so you must avoid using plastics which can easily shatter, a hinge lid will open releasing the pressure if the barrel gets stuck with no consequences for the person who uses it, a threaded cap won't let the pressure go out, even most of the canisters can easily hold the pressure it's recommended to use one with a hinge lid.
The bigger the canister it is the smaller the explosion will be, this is because alcohol requires a small volume to quickly rise up its pressure and temperature (Ideal gas law), the optimal volume is around 15cc and 30cc, to calculate its volume put the canister in a scale and zero it, the weigh of the water in grams will equal the volume in cubic centimeters.
How to chose the right barrel: I can't recommend an specific barrel because It depends on the size of the canister you are using, when choosing the tube remember the alcohol power has limitations, If you use a wide tube it has to be short to avoid the suction of the gases cooling, if it's thin it has to be long to use all the energy, with this said I would recommend a barrel which has an inner volume between 1/2 and 2/3 of the canister volume.
A long and thin barrel will shot small projectiles at a bigger speed, a short and wide barrel will shot bigger projectiles but more slower.
Some electrical tape helps to hold the piezo in it's place.
To shoot it put the palm agaist the lid, hold the device with the other hand and press the piezo, a spark will fly between the screws making alcohol to burn, since the volume remains constant the temperature will rise causing a small and controlled explosion shooting whatever it is inside the barrel.
I use 96º alcohol, two drops are enough to power it for 2-3 shots.
TIP: the alcohol needs time to evaporate and mix with the chamber's air, you'll notice it won't shot until few seconds have passed since you closed the lid, if it still doesn't shots heat the canister a bit with your hands, if still doesn't works make sure the spark flies correctly between the two screws, if nothing of this works the problem might be the piezo is too wasted and doesn't produces a spark dense enough to lit the gases.
Don't use any other fuel to make it work, I've tested with HHO and it worked fine but other canisters might not be able to stand the huge pressure of an HHO explosion, don't even bother with kerosene (lighter fluid), gasoline or other similar fuels, they just don't burn.
Be responsible, don't aim or shot at animals or people, If the barrel get's stuck the pressure will open the lid and scape causing no harm to the person using it.
I'm not responsible for the misuse of this information.
Thanks for your time.