So, I had built a potato Cannon 7 years ago. I still use it but , I thought it would be cool to build a compact one. Then , I saw a couple of mini or micro guns meant to fire air soft pellets, on the Web. I then thought of building a Mini Potato gun rather than simply building a compact version.
I have compiled a large picture Library of the build. BUT as always.... I'm designing and building at the Same time. The first gun worked, but not well enough.
The second gun/build works awesome but Is not exactly the same as some of the pictures.
Chances are that if you are on Instructables and can't figure it out.... You are 10 years old and probably shouldn't be playing with fire anyways.:)

I find most Instructables projects to be a guidline anyways and so I want anyone trying this to realize that it's up to you to fine tune this to your liking.
Also!!! There is a chance of minor electrical shock... Explosion and or Fire when dealing with this project!    It is up to you to use your judgement and take all safety precautions seriously!!!           Even when working with any tool required.    Follow the manufacturers instructions!

Step 1: Parts List

Things U will need.   1 1/2" ABS pipe. (Hint) A 12' length is close to the same price as a 6' or 3' piece. I brought a cheap hand saw to the hardware store and cut it in half in the parking lot so it would fit in my car.
You will need ABS pipe cleaner. I use Isopropyl alcohol which is cheap!
You will need various ABS "Fittings".   My Final Build consisted of :
1 ABS threaded plug (Nibco 5818-cs)
1 ABS Threaded Cleanout to 1 1/2 couple
1 ABS 1 1/2 double coupling.
4" of standard 1 1/2 " ABS pipe.
1 ABS permanent Cap (Nibco 5817)

I would recommend using :
1 IPEX 027341 cleanout. This eliminates the use of the double coupling.
Then just add a little to the length of the 1 1/2 pipe.

I used thin wall aluminum tubing purchased from the hobby store for the barrel.
It measures 20 CM long by 7mm inner diameter.
You need a can of ABS solvent cement.
I used "Oatey" ABS Cement premium grade yellow. NO. 31500.
I used a variant of "Crazy glue" to glue the barrel into the cap. You could use PL construction adhesive, JB Weld or another epoxy if you want. Apparently even the ABS cement will work as it melts the ABS for a tight seal.

You need some sort of propellant/fuel. Hairspray, wd40 ,Brake clean, Butane, Propane, Isopropyl Alcohol, Deodorant spray etc. etc.

I added a tube to the rear cap made from Lamp parts.   I did so to be able to add a Gasified propellant. (eg. Propane.)
Propane works great for my 6 foot long potato cannon, Not so well for this. DON"T BOTHER WITH THE TUBE. :)

The First "Baby potato gun" had a piezo ignition system from a BBQ Lighter.    I found that it was very unreliable and sometimes was not enough spark to ignite the fuel.

I used 2 M4x40 bolts with 4 nuts to construct an adjustable ignition gap.

I purchased a SUNDE Model SD-E5 electronic BBQ ignition system. It was $25 but can fire 3 sparks at once and does so repeatedly at 12 kilovolts.   (Downside is that it's a large, battery powered step up transformer and it costs just as much as the project does....)

You will also need some wire, some rosin core solder, electricians tape or hot glue, and automotive crimp terminals can be helpful.

Step 2: Tools!!!

There are many ways to do this but here is a list of basic tools required.
1- A hand saw eg. Hacksaw.
2-A measuring tape
3- Sand paper and a file
4- A drill and or a Dremel/Die grinder to make holes etc.
5-set of drills aka drill bits.
6-screwdrivers, one flat and one that fits the bolts you will use for the spark gap.
7- a couple of pairs of pliers and or wrenches.
8- Wire cutter/stripper/crimp-er combo.
9- a soldering iron (may not be required)
That's me on the right.....
BTW these pics are from different builds BUT the tools used are the same.!!!!

Step 3: Start Building.

This is one of the end caps. Drill a small pilot hole directly in the center. Then use a larger drill bit to make the final diameter hole for the barrel. I used a 1/16th pilot hole. then slowly stepped up to a 19/64ths drill, which is slightly smaller than the barrel. If you continue to slowly "bore" the hole with the drill you can get a nice tight fit. (Here is where a dremel Tool is a benefit.) You might find different size tubing and so the drill sizes listed might not work. Basicly start with a small hole and gradually go bigger.

Step 4: Cut Your ABS (expansion Chamber)

Here is a Pic of 2 pieces of hand cut ABS Pipe. The one on the left is a BAD cut. The one on the right is a correct cut and was finished with a sander. Be sure to make the edges square and clean any rough edges. Also.. before gluing ... Clean thoroughly with ABS cleaner.... I find Isopropyl Alcohol is cheap and works just fine.

Step 5: Barrel Considerations

The Barrel I purchased was a bit long so I cut it with a dremel , you could use a pipe cutter or even an Olfa knife blade.
Next you want to lightly chamfer (Taper) the edge of one end of the barrel. This is to make it easy to cut into your potato ammo.
DRY FIT EVERY PIECE BEFORE GLUING IT!
You can do this step (Chamfer) with a file or sandpaper if you don't have a dremel.

Step 6: Installing the Spark Gap Electrodes.

The Cap and barrel assembly will be permanently glued to one end of the 1 1/2 pipe so you need to install the electrodes as close to the other side as possible. This will allow you to make adjustments later. In this picture you can see the small cap screws and nuts used . You can also see the "Cleanout fitting" which is glued to the ABS Pipe and has an internally threaded end to accept the cleanout plug.

Step 7: Drill Holes for the Electrodes

This is the 1 1/2" "cleanout fitting viewed from the side. You need to drill straight through the ABS pipe as close to this joint as possible ,leaving enough room to allow for wire connections. The second pic is of the piezo ignition from the first build But the electrode installation is the same.. You can see in this Pic how automotive crimp fittings can be used for the electrodes.

PS. I'm sure this piezo spark method would work fine BUT it was unreliable.... especially seeing how easy it is to get the Air fuel Mix wrong in such a small expansion Chamber.

The plus of having these adjustable electrodes is that you can use all sorts of spark igniters.

Step 8: Finishing

Once all parts seem to fit together nicely, I can't give much more instruction as to ABS Cement except (Follow the directions!)
Quickly apply a light coat to both surfaces to be glued. Don't take your time! If you don't get it right the first time you can't save your work. The Cement says LOW V.O.C. This is so untrue! In fact , the vapors from the glue were enough to shoot my first couple of potato "bullets".

I have found that "Brake Clean" works the best. (Brake parts cleaner)
Cut a potato in half length ways. Poke the barrel through so that the "Skin" is the tip of your bullet.
I used a wooden Skewer to ram the potato down to the bottom of the tube.
with the cleanout unscrewed I shot a short burst of Brake clean into the end from a distance.
Then I screwed the end cap on and hit the sparker. (in this case it was the BBQ spark ignitor.)

Good luck with your builds!!! feel free to experiment with different barrels and different sized expansion chambers/fuel types.......