A small, low-power spud gun made from a plastic bottle, a metal tube from a mop handle, and a barbecue lighter.

Disclaimerama: This is probably the most dangerous of my Instructables to date. You are improvising a firearm that uses a surprisingly large gas explosion in a flimsy plastic enclosure to propel roughly 80 calibre projectiles down a barrel that may be nearly two feet long. Just a few of the risks inherent in this project are:

- Injury from flying projectiles

- Injury from shrapnel if the gun explodes (unlikely but possible)

- Very, very loud noise if the combustion chamber explodes- it's noisy at the best of times but if the bottle bursts you have every chance of going permanently deaf

- Burns

- Setting yourself on fire if you have any residual fuel on your clothes, skin or hair

- Cuts from the torn metal edges

- Electrocution from a roughly 50,000v source (the electricity is safe for most adults but the shock/surprise can cause accidents)

- Prosecution according to local law enforcement if your behaviour with the gun is deemed to be unsafe, or if possessing such a weapon is illegal in your area

- Small curious children asking you endless questions about how you built it.

If you aren't willing to take all of those risks, don't do this. I accept no responsibility for whatever happens to you no matter how many safety precautions you take. If you injure yourself firing one of these without wearing eye and ear protection or injure someone else by using it in an unsafe manner, then I will probably see you in the next Darwin awards compilation.

That said, when used properly this gun is a lot less risky than a PVC spud gun. Where an impact from a whole potato from a traditional gun could cause nasty injury, this small-bore gun fires a tuberous projectile about the size of a wine cork- I wouldn't want to be hit in the face with one, but a body impact through clothes will in the worst possible case be as bad as a paintball, and people get shot with those for fun. If you fire heavier projectiles than bits of potato or use any fuel more volatile than deodorant or hairspray, you're on your own.

Step 1: Requirements

You will need:

- A plastic bottle. I used a square-section 1 litre bottle that used to hold [http://en.wikipedia.org/wiki/Squash_(drink) squash].
- A lighter with a piezo igniter, not a flint wheel, preferably a large barbecue lighter rather than a pocket cigarette lighter. Small lighters will do but make the electrical part slightly more fiddly.
- A long metal tube. I use the handle from a cheap mop bought at a supermarket ~~cut~~ broken in half for this build, an entire mop handle for the previous one and part of a towel rail for the first one I built. The tube should be about 2 cm (3/4") in diameter and as straight as possible. Wonky tubes make poor gun barrels.
- Solid core wire, duct tape, electrical tape, assorted plastic junk- basically your average maker stuff. If you don't have any, buy it- it's always useful.
- Craft knives or similar implements
- A soldering iron and glue gun would come in handy but as they are for fixing stuff together can be substituted with MORE TAPE

Step 2: Reverse Engineering

in which the intrepid builder turns a consumer product into its constituent components, thus reversing engineering.

The lighter will almost certainly not use screws, as using screws encourage disassembly. Use a judicious combination of ingenuity and force to open the outer casing, trying not to puncture any butane-containing components. Once you have opened the lighter slightly your fingers are better than a knife, more sensitive and less likely to puncture the butane tank. If you do puncture the butane tank, leave the lighter outdoors somewhere until it has emptied- butane is heavier than air so will pool in a room and a tankful will present an explosion hazard. Upon opening the lighter, take a moment to survey the ingenuity of the trigger mechanism that some unsung genius has designed and rarely gets recognition for. If you are saving the mechanism, note how it fits together.

Remove the piezo and test on a nearby piece of metal for spark length. Shock yourself with it if you must- it will only make it less surprising when it inevitably happens during use of the gun. Piezo shocks are mildly surprising but actual cause a negligible fraction of the pain of a camera flash capacitor shock.

Step 3: Metalwork

Approximate the midpoint of your tube. Mark if you wish. One end of the tube will probably have a screw attachment for the mop head firmly attached into it. As long as the other end is clear you don't need to remove this and it saves you having to cap another sharp metal edge. Put the midpoint of the tube over your knee, grasp the tube about a foot away on either side, pray and pull firmly. If all goes according to plan, the tube will kink with minimal damage either side.

Bend the tube a few times, enjoying the feeling of bending a metal bar with just your hands, then reverse the bend until the tube snaps. The plastic coating may disguise when this happens, so be vigilant for the snap noise.

If the broken ends of the tube are squashed too flat you will need to open out the end of the half without the screw attachment- this will be your barrel. The best way to open it out is with a strong screwdriver and some elbow grease. Look down the inside of your tube and pretend it's a kaleidoscope.

The open end of the barrel should be the same diameter as the rest, but the tiresome mop manufacturers insist on crimping it inwards slightly. Use the trusty screwdriver again or a solid conical object such as a pair of pliers to flare the end of the barrel to match the rest of it. Your tuberous projectiles will be a loose fit unless this vital step is completed. Try not to breathe in any of the metal dust generated by flaring the tube, there are more fun things in life than having itchy lungs.

Step 4: Bottle It

Fire up your skanky old soldering iron, covered with rust from before the days when you appreciated the true necessity of tinning an iron tip. If you don't have one, fire up your clean new soldering iron and hope you can clean the plastic off it afterwards. Mark a circle the size of your barrel on the bottom of the bottle.

Using a judicious combination of patience and the soldering iron, open out a circular hole just smaller than the barrel. Test fit the barrel, slightly enlarge the hole and repeat. Patience at this stage will pay off as a bad gas fit around the barrel seal will decrease the ultimate performance of your gun. Once the hole is a snug fit around the barrel, let it cool (without the barrel in it!) until hard. Try to make the hole "straight" through the plastic, it will make your life easier later on.

Heat up the end of a needle or a piece of your solid core wire in a lighter flame and push two holes about 1cm apart near the neck of the bottle. These are the holes for your electrode wires - they should be within a finger's reach from the neck of the bottle to later facilitate adjustment.

Step 5: Wire It Up (this Is Familiar...)

Strip two pieces of solid wire long enough to stretch from the holes to where you intend to mount your piezo- allowing plenty of extra length leaves you space for multiple strain reliefs (pieces of electrical tape). Strip at least 1cm from one end of each, and bend these ends at a sharp right angle. Insert the longer stripped ends into the electrode holes. Arrange the wires so that the tips are 5mm apart or closer, and firmly tape (and preferably hot glue) the ends in place.

Attach the other ends to the piezo. Solder is not necessary at this point, one of the principal joys of working with piezo sparkers is that the spark can jump millimetre gaps in wiring so contact joints are perfectly good enough. This is also one of the principal frustrations of working with piezo sparkers- several types of tape are considered a conductor by the spark, including possibly duct tape. When possible use electrical tape, heat shrink or solid plastic to insulate the wires.

Firmly attach the piezo to the outside of the bottle. If you are using only the element, you may want to consider how to insulate the two wires from eachother to prevent it sparking over.

Realise the flaw in your plan- plastic bottles tend to deform under lateral pressure such as is required to operate the sparker button. Improvise and attach some long rigid object underneath the sparker to distribute the load- what else but the ubiquitous bamboo skewers? Realise this isn't much better, but in the absence of PVC pipe it will have to do.

Step 6: Home Stretch

At this point the "Instructables mist" may descend as you realise the end of the project, like the bottom of the ice cream tub, is in sight, and power through to the end without properly documenting your progress.

Insert the tube without any plastic bits in it (hereafter the "barrel") into the bottle from the neck end, with the non-broken end downwards. Push it most of the way through the hole until about 10cm (4") is inside the bottle. If the barrel is not quite straight, don't worry. If the barrel is not at all straight, as mine turned out, you can soften the plastic around the barrel seal by wafting a lighter flame at it and straighten the barrel out. If you happen to have a hot glue gun, run a bead of hot glue around the seal once you are satisfied with its straightness. If not, get creative with the duct tape to find the best way of making it conform to a cylindrical 90 degree join.

Cover the sharp pointy end of the other tube with some suitable covering to prevent injury on it. Do not underestimate the power of torn metal edges- I stabbed myself in the finger with the Marksman in the picture and didn't bleed, then promptly cut myself open on half a mop handle.

Duct tape the non-barrel tube (hereafter the "barrel support") to the bottle (hereafter "combustion chamber" unless I get bored with typing that) pointing parallel to the barrel. Resist the temptation to try to get everything perfectly straight at this point- tuberous projectile ballistics is not an exact science.

Find your junk box. Take your junk out of the box. Find some sort of rigid support from your plastic junk to attach between the barrel and barrel support. I used a bracket from a bike pump that didn't fit onto my bike, as it already clipped onto the barrel support and merely needed taping to the barrel.

Step 7: Gun It

Congratulations, you are now in possession of a "Small Bertha" model small-bore potato rifle. To test fire:

Put on your eye and ear protection. Crumple up a piece of paper to a sphere that will fit snugly but not too tightly in the barrel. Load it and stoke as far down the barrel as you can. Open the bottle cap and prime with a short blast of your chosen fuel (a short blast- quarter of a second is probably too much from a normal aerosol can). Screw the lid back on, and turn the gun upside down a few times to mix the fuel. Grasp the gun firmly, holding it against your body to prevent recoil. Click the sparker and expect a very loud bang- expecting it stops you dropping the gun and flailing like a mad person when it eventually does make one.

If nothing happened, turn the gun upside down again, wait a few seconds and try again- if this doesn't work a few times your fuel mix was probably too lean, or possibly much too rich.

If there is a slow "whoof" noise with a visible flame front and the gun becomes hot, your mixture was probably too rich- use less fuel next time. This can also be caused by inadequate mixing of fuel and air- if reducing the fuel quantity results in no firing at all, try the previous quantity of fuel and rotate the gun for longer to provide more complete mixing.

If there is a sharp bang with a barely visible blue flame, your fuel mix was about right- the paper will probably go quite a long way.

Open the bottle cap with the barrel pointing straight upwards and let the combustion chamber vent for a few seconds. Put your lips to the neck and blow (as if you were blowing on a fire or hot food, not your cold hands) for a few seconds to further vent the gun. Repeat this from the muzzle. If you are going to fire again, let the gun cool for a few seconds- hot plastic is weak plastic. If you are going to leave the gun, leave it standing with the muzzle pointing upwards and the bottle cap off to let it air out.

Step 8: What Next?

To let your Small Bertha spud gun earn its name, hold by the barrel and press a potato onto the muzzle to "cookie cutter" out a potato bullet. (You might want to remove the last 6 inches or so of plastic coating from the muzzle to do this.)

Stoke down to nearly the other end of the barrel (stop before the crimped part where the cross-section of the barrel changes shape). Load and fire the gun as above, making sure to fire in a safe direction at a solid backstop or outdoors, away from people, animals and yourself. Don't fire hard or overly heavy projectiles. Don't even think about, say, taking the ball out of a ball mouse and firing that.

Where to go from here? Build a bigger one, of course. See if you can integrate the butane tank from the lighter into the design so you can fuel it with the press of a button. Use an entire mop handle to make a very long barrel.