I thought this up after watching the Mythbusters make a steam powered machine gun. This is a fun little project that lets you shoot potato across the room with a nice pop. It costs about $10 to $15 to make depending on copper prices and parts already in your garage.

A grill lighter is disassembled and used as the heat source for the steam production along with some copper plumbing parts and 1/2" copper pipe. Some water is placed inside the barrel, a potato plug is inserted as an air seal and projectile, and a cork is used as a sudden pressure release. The flame from the lighter heats water to steam and the pressure increases in the barrel until the cork and potato can't hold anymore, then it pops and launches the potato to wherever it's pointed.

This is a prototype steam potato pistol and can be taken a lot further, but I'm just sticking to functioning basics here. Someday I would like to make one with a pressure gauge, quick release valve, and a better handle with the fuel stored inside the handle. I'm calling this version the SPPP1.

It is a little dangerous to build and use because of the flame and hot surfaces, flying objects, and steam; be careful. Playing with fire, pressure, and projectiles is always risky, even on this small scale. Don't shoot an eye out, I'm not liable for any injuries you incur with this. ;( What happened to that "not liable" category of Instructables anyway?

Tools:
Soldering torch
Pipe cuter
Jigsaw
Drill
Pliers

Parts:

Various short wood screws
Some leather strips from an old belt to hold the fuel bottle
Some scrap wood for a handle
Lead free plumbing solder and flux
4" length of 1/2 inch copper pipe

(1) 1/2 inch copper pipe cap
(3) 1/2 inch copper ST 90 degree elbows
(1) bag of copper plumbing pipe brackets
(1) candle/grill lighter
(1) Potato
(1) Cork

Step 1: Start Plumbing the Boiler and Barrel

Cut about 3.5 to 4 inches of 1/2 copper pipe for the barrel. After cutting the pipe make sure to file down the lip on the inside of the pipe formed from the pipe cutter.

Attach the three elbows together at the end of the barrel to form a curl and put a cap on the last elbow.

Dry fit all the parts as shown in the photo. Make the elbows form off to the side so a flame can heat water inside the cap.

Step 2: Sweat Those Pipes

Sweating pipes is soldering. Sweat the whole assembly together. This part took the longest for me because I have limited plumbing skills. One of my fittings took eight tries before I finally got an air tight seal. The key here is to make the joints completely air/water tight like real plumbing. After soldering, cool the barrel and blow into it with your mouth; there should be should be absolutely no air hissing sounds.

Gdawg at Instructables has a nice tutorial on How to Sweat (solder) Copper Pipe

Step 3: Make a Handle

This is a quickie handle that I'm showing in the pics. There are so many cool possibilities with this. Just cut some wood at angles and use the plumbing brackets to hold things together. Notch the top of the handle for the barrel to rest into. You will have to custom bend the brackets to make things fit nice. Also pre-drill the holes before inserting the screws.

Step 4: Attach the Fuel System

Get one of those long lighters and open it up. Pull out the trigger parts, wires, butane bottle, and hose with the metal tip. Not all lighters are same; in mine I had to keep a plastic slide switch that opens the fuel valve. You may have to make yours a little differently to function.

Bend a pipe bracket and attach it to the side of the handle so that the flame will burn just under the copper cap or boiler.

Insert the piezo clicker into the handle to make a trigger; this is what makes the spark to ignite things. Ground the black wire to the barrel and the long white wire gets routed to the ignition point under the boiler.

Screw a piece of leather to the handle and wrap it around the back to the other side. Fit the butane bottle in there and attach the leather again with a screw. This should hold the tank snuggly, but also let it slide a little. The slide switch to open the fuel valve hangs nicely on the leather strap. To turn on the gas you just push the butane bottle down a little until you hear the hissing; to turn it off you slide the tank up.

Now route the fuel hose and metal tip to the newly attached bracket under the boiler. Wrap the white spark wire around the base of the metal tip. Attach it in the hole with a twisty tie or something and use pliers to bend it so the flame base will be 1/4" under the boiler. The spark should jump from the metal tip to the bottom of the boiler, if it doesn't adjust the bracket until it happens.

See the pictures on all the above stuff.

Step 5: Add Water, Plug, and Shoot.

Add 1/8 teaspoon (about ten drops with eyedropper) of water into the barrel and rotate the whole thing upside down to route the water into the boiler.

Cut out a potato plug using an extra piece of scrap pipe and insert the plug into the barrel. This should have an airtight seal and want to push back out, it should not go far into the barrel.

Wedge the cork into the barrel; don't overdo it.

Move the butane bottle down to open the fuel valve.

Click the piezo clicker to ignite the flame.

Point and wait about 1 to 2 minutes.

POP! The spud should fly; mine has a range of about 25 to 35 feet.

If it takes much more than 2 minutes to pop, then turn off the flame and let it cool off for a while; either the air leaked around the potato and cork or the cork was in too tight.