How Do I Build One? The Ignition System Perhaps the most important and difficult part of the gun to construct is the ignition system. I tried the cheaper piezoelectric (mechanical) BBQ sparkers but they just plain sucked. For the price of two of those I was able to get an electronic igniter designed for a gas heater. I had to modify it slightly and make it fit inside a 2" pipe but that turned out to be fairly simple and the spark is 100% reliable. That's what matters. Go to your local propane retailer and look at their gas heaters. One of them will almost certainly have an electric igniter rather than the cheaper mechanical type. Get one of those! The one I got had one lead from the bottom and one on the top that was intended to ground to the heater frame itself. I soldered a piece of 18 gauge 40Kv wire to that lead which then gave me two wires to attach to the brass bolts. I found out the hard way how important it is to use high voltage wire. The stuff is two or three bucks a foot but well worth it. I started out using regular 300 volt "hookup" wire and quickly fount that when the leads got within a quarter inch or so of one another the spark would jump straight through the insulation. I then ordered some 18 gauge 40Kv wire from Surplus Sales Of Nebraska and rebuilt the igniter with the new wire. That ended the leakage problem and I think the spark even looked brighter and lighter blue (whiter is hotter). Getting the igniter to fit inside the two inch pipe handle was a little more difficult. I got a 2" connector (connects two pieces of 2" PVC pipe), drilled a hole in the middle of that, notched two short pieces of 2" PVC to fit around the button stem of the igniter, positioned the igniter in the connector and glued the two pieces of PVC pipe on either side of the igniter. Then I glued a cap on one end (a plug would have worked as well). Next I used some foam filler (Great Stuff) to fill the pipe and hold the igniter in place. A little hot glue around the button stem will keep the foam from leaking out of the hole and making a mess. After the foam had fully cured I trimmed it up and plugged that end with a 2" test cap (see image) in which I had drilled two small holes for the wires to fit through. Lastly I glued a 2" female connector to the bottom so that I could screw it on to the 4" x 2" Wye where I mounted the male connector. I chose 2" #6 brass bolts for the spark system. I drilled two sets of holes slightly smaller than the bolts, one set at each end of the 4" x 2" Wye. There is little solid evidence that a second spark will help performance but it probably couldn't hurt either. I put washers inside and out to help seal the holes and distribute the pressure on the PVC. The leads from the igniter were then bolted to one front and one back bolt and the other two bolts were connected with a short lead (see image). This method wires the sparks in series which keeps the voltage high rather than splitting the voltage by wiring them in parallel.

The Main Gun Body The gun body consists of several PVC pipe fittings connected with short pieces of 4" and 2" PVC pipe (from left to right in the image below): 4" Cleanout 4" x 2" Wye 4" x 2" Tee 4" to 2" size reducer 2" Male connector   In the image above the red fitting on the right end of the gun is simply a 2" female connector I use as a thread protector when no barrel is attached. This design was essentially ripped off from this NoDice.org web page. (See the "Version 3 Advanced Cannon".) I made a number of changes (igniter, front end size changer, sex of the barrel connections, fuel, etc.) but I really liked the overall design so I adapted it to my particular tastes. On the 4" x 2" Wye I attached a 2" Male connector with a short piece of 2" PVC pipe. This fits the igniter's 2" Female connection. On the 4" x 2" Tee I attached a short piece of 2" PVC pipe and a 2" cap to make a short handle. I also filled this handle with the same foam filler I used on the igniter just to insure that if there were a structural failure it wouldn't be right where I held on to the gun. If you've never done any PVC work get someone at the hardware store to give you a quick rundown and get you the right surface prep and glue. I used Schedule 40 PVC for my gun. Schedule 80 is harder, thicker and stronger but when it bursts it makes much nastier shards than Schedule 40. This design is double thick except at the front of the combustion chamber so it's very unlikely you'll blow it up unless you start injecting exotic fuels like Acetylene and pure Oxygen into it.

The Propane Fuel System Attached to the side of the gun is a piece of 3" PVC pipe which just happens to fit a propane torch perfectly. I used epoxy to glue two 1" x 2" magnets to a 3" test cap and then covered the magnets with a piece of self-sticking felt (like you'd use to cover the bottom of a table lamp). Then I epoxied the test cap into one end of the 3" pipe. The magnets hold the tank in place and the felt keeps the magnets from being broken when a fresh tank is inserted. The straps that hold the tank container to the gun are just big hose clamps. I wanted to keep the hose clamps from tearing up the paint and from grounding out the ignition system (the front strap is practically on top of two of the igniter bolts). So, I found some soft black rubber hose and with the help of a little soap I was able to push the hose clamps through the hose. I also put a short piece of this same black rubber hose on the tip of the torch to increase its size slightly so that it fit into the 5/8" hose snugly. Adding a little PVC thread grease allowed me to turn the tip without removing the hose so that I could adjust the air/fuel ratio on the fly. I've found that adjusting it all the way out (leanest mix) seems to give me the best results but without a chronograph it's hard to tell for certain. At the front of the gun, just below the barrel connection, I used a couple of cable ties to make a loop to hold the 5/8" hose in place. From there it goes to the brass ball valve assembly. The Brass The ball valve assembly consists of a 5/8" barbed hose fitting, a 1/2" ball valve and a 1/2" street elbow. Use lots of Teflon thread tape on the threads to make air-tight seals. The ball valve assembly is attached to the gun by screwing a 1/2" brass nipple into the PVC. A 1/2" brass nipple is about 1" long with threads that go in opposite directions and meet in the middle. Screwing one of these into a piece of PVC can be a tricky procedure and really has to be done right to prevent the brass from being blown out of the gun. This procedure worked well for me: Drill a 3/4" hole at each end of the gun (one for the exhaust cap and one for the ball valve). Center the holes as far from any seam as possible. Put Teflon thread tape on one end of the 1/2" nipple and place a cap on that end of the nipple. The Teflon tape will make it much easier to remove the cap when your done. You may want to ream out the top edge of the hole very slightly to help the nipple get a good bite. Apply some slow drying Epoxy to the end of the nipple and the hole. A little goes a long way. Using a long handled wrench screw the nipple (with the cap on it) into the hole. Be sure it goes in as straight as possible. Turn it until the end just barely breaks the inside surface of the PVC fitting. That should be almost exactly 1/2". Remove the cap and clean any excess epoxy and Teflon tape from the threads. Let it dry for 24 hours before putting any real stress on the fitting. You should now have two 1/2" brass nipples protruding about 1/2" out of each end of the gun on the left side (see the images). Once the epoxy is fully set you can screw on the ball valve assembly. Again, use Teflon tape to get a good seal. Wrap it around the threads in the same direction you'll be screwing on the fitting and use five or six wraps. Too much and you won't be able to screw the fitting all the way on and too little will not make the air-tight connection you want. You'll notice that I've angled my ball valve slightly downward because the 5/8" hose will come from beneath the Tee handle. The Barrels The barrels are dirt simple to make but determining the optimum length is slightly more difficult. Just attach a 2" female connector to the end of a 2" piece of PVC pipe and you have a barrel. I prefer the 1 1/2" inch barrel because most potatoes can be cut length ways at that size and thus make better projectiles. To make a 1 1/2" barrel you have to use a 1 1/2" to 2" size converter to size down from the 2" female connector. You can likewise make a 3" or even 4" barrel using size converters. Optimum Barrel Length Using the right length barrel will give you the best performance. In general the rule is that the barrel should have roughly 3/4 of the volume of the combustion chamber. To figure the volume of any cylinder just use the formula pi * radius * radius * length. This is how I ended up with a 7 foot long 1 1/2" barrel. With a 4 liter (244 cubic inch) combustion chamber I needed a 3 liter barrel. If you run the numbers that comes to just under 7 feet for the 1 1/2" barrel and a 2" barrel needs to be just over 4 feet long. Here are a couple tools that I've used to figure the volume of my combustion chamber and thus the optimum length for my barrels: SpudTool v1.0 from AdvancedSpuds.com is great. RatCalc from Burnt Latke is simpler but also kicks butt.

A Crucial Modification Aiming from the crotch can be a bit tricky but having a golf ball hit a tree and come screaming back at you is down right dangerous. Shortly after that brush with death I went to my local gun range, The Eagle Gun Range and the guys there helped me pick out a suitable Weaver rail and BSA Laser Sight. I had to shorten the rail slightly to make it fit before I bolted it to the top, front edge of the BFSG. I then mounted the Laser Sight and, through sheer luck, the remote control button reached exactly where my thumb rested on the Tee handle. I fastened it in place with a cable tie and put the Velcro pad just where my thumb rested and I was set.

I can not stress enough how important this modification is for safety as well as preventing wasted shots. I used it for the 3/4" plywood test (see The Gallery Of Destruction). During that test I didn't miss any shots or hit any hard surfaces that might have sent a golf ball back at us. It's not much use in full daylight but if you can find some shade or are shooting on an overcast day it works quite well. In addition, the use of a Weaver style rail allows me to easily move the Laser Sight to another platform. The more modular the design the better I like it.

 

Spud Cutter To make it easier to cut spuds to fit in the barrel you can use a simple device made of a sharpened piece of PVC pipe and a PCV pipe coupler. Just cut a piece of PVC pipe about six inches long and bevel the outside to a sharp edge. Glue that into a PVC coupler and ream out the other side of the coupler so that it fits loosely on the barrel. A Dremel tool will save you a lot of effort here. You can also put a very slight bevel on the inside of the pipe so that the potato slug is slightly oversized making for a tighter fit in the barrel. To use the spud cutter just push a spud into the cutter so that it makes a good slug, place the spud cutter on the end of the barrel and use a ram rod of some type (broom handle, etc.) to push the spud slug from the cutter into the barrel.

Tips and Tricks Any threaded PVC to PVC connection that you might ever want to take apart again should be treated with white lithium grease. You'll find it in the plumbing section. Without it these threaded connections will be extremely sticky. You will find a pair of rubber strap wrenches very useful for unscrewing stuck PVC parts without damaging them. Several companies make paint specifically for plastic. I used Krylon's version called "Fusion" and it worked extremely well.

Other Useful Links Burnt Latke is a great source for ideas and test data to help you optimize your spudzooka. The Spudgun Technology Center has some useful information and VERY cool parts like Clear Rifled Barrels! AdvancedSpuds.com has some good ideas for several aspects of spudgunning. I stole the idea for my pre-mixed propane injection from the Purple Pounder site. (Here's a picture and another one.)