A Pneumatic Potato Gun Page

Home     Disclaimer     How it Started    Op Theory     Construction

How it all started..

"A some point when my wife was describing my adventures with the potato gun to  some friends, the term 'waste of time' was used.   I hate when people say that." 


     Testimony to my condition which I call "chronic, untreatable youthfulness" (CUY) is the reaction I have when
     talking a group of my neighbor's about hometown politics at the annual block party, and then seeing an 11 year
     old out of the corner of my eye with a backpack mounted, high pressure "Super Soaker" squirt gun. One minute
     I'm engaged in a discussion on some local building inspector's policies, next, I'm distant thinking, "Cooool! I bet I
     could make one of those!!!!"

     But I don't think you have to really have this condition to be intrigued by simply the name "potato gun". The very
     term invokes curiosity; with two words not ordinarily seen next to each other. They surfaced on the VW
     newsgroup for some strange reason, many people chiming in about how fun and potentially dangerous they were.
     And speaking of, there are two more phrases that grab the attention of my condition: "fun" and "potentially
     dangerous". Perhaps it is that they always intrigued you, but suddenly in life you find that there are no more
     parents around to tell you can't do something, and you can now just hop in your car and drive to Home Depot
     and buy stuff.

     This page chronicles how I come about building my potato gun, without all the technical details. If you are
     familiar with my writing, it is more of a story. And it is long. 

     In an idle moment one day, I did an Infoseek search on "Potato Gun" and got plenty of hits; proving to me once
     again, that there is nothing that isn't on the internet. I ended up on page called "Backyard Ballistics". Pretty cool
     stuff. I started considering building one of the "hair spray" guns, but then I saw a link to the page for the
     "Pneumatic spud gun". I had to read the text and stare at the diagram for many minutes before it all made sense.
     Then it seemed waaay to cool not to try. I have a compressor and any use that I can find for it only serves to
     justify its purchase. Any use. I forwarded the URL to my partner-in-waste-of-time-projects, Bill, and called him.
     He suffers from CUY as well, but not as severely as mine. He closed the conversation saying "When you build
     that thing, give me call. I want to come over".


     I made a mental note of the main parts and a few days later even wrote "PVC" on my shopping list prior to one
     of my Saturday pilgrimages to Home Depot. But I intentionally passed up the "plumbing" isle. "Too much stuff
     going on right now, too many other projects. Maybe another day". But a few weeks later, may brother-in-law and
     his large family came to stay with us for a weekend. Their arrival was considerably earlier than expected and I
     had planned to do yard work that Saturday. I spent a few hours working in the yard but began to feel a bit guilty
     at not spending time with my wife's young nieces and nephews, and our own, curious 6 year old son. For some
     reason I went into the basement to get something. Out of the corner of my eye, I spotted high above on a rack, a
     long length of 1 1/2" PVC. The exact diameter specified for the barrel of the potato gun and about a foot and a
     half longer. I had no intention at that point of building the full-fledged gun, but thought that just applying
     compressed air to the end of the pipe with a potato shoved in there might make for some amusement. I rifled the
     "plumbing" box in the basement and came up with a cap and some glue. I glued on the cap, then drilled a hole in
     it and put a compressor fitting. I cut about a 3" piece of the PVC off the end and with the bench grinder ground
     a sloping bevel on the circumference of one end, producing a sharp edge.

     My 10 year old nephew strolled into the garage looking inquisitively at my hands all covered with white PVC
     "dust". "Drew, go tell Ryan I'm making the potato gun" I said, Drew strolled back out. A few minutes later I
     rounded the corner of the garage with the seven and a half foot spud delivery prototype, with a blow gun affixed
     to the capped end and compressor line attached . I met Ryan running up the yard. "Daddy, is that the potato
     gun??" I was distracted by the sound of a car door closing on my driveway. It was Bill, coming over to pick up a
     tool he had left at my house. "Boy Bill, your timing is perfect. Ryan, run in the house and grab the bag of
     potatoes in the hall closet..!" Bill walked up the driveway with a big grin on his face.

     It was reminiscent of a Disney movie about the mad professor stepping out into the sunshine with some wild
     contraption, kids scurried everywhere, I spotted the neighbor's girls peering out of their open bedroom window. I
     felt like "Doc Brown" from Back to the Future. You could feel the excitement building. Ryan returned with the
     potatoes, "There's only three left daddy!?". "Ok", I thought, "we have to make the best use of them". Placing the
     beveled 3" piece on the top of one of the spuds its end, I forced it down over the veggie until it touched the
     bench. Then I pushed the "core" of the spud out of the PVC. A perfect potato cylinder. I quickly filled a small
     bucket with some warm water and dropped the 3 cut potato cores into it. "Which way you gonna shoot it?"
     someone asked. "I'll aim it up over those trees, nothing but woods that way" I replied. I had conservatively dialed
     in 20 psi of pressure on the compressor regulator, not knowing what might happen. I honored a neighbor kid by
     letting him use the broomstick to push the potato core down into the barrel as far as the stick could push it. "Here
     we go" I said and squeezed the inline blowgun trigger...

     There was a hiss and the harmonic sound of air rising in a tube. In what seemed like about a minute and a half
     later, the potato made an appearance at the top of the barrel, rolled over the lip and fell to the ground. One of the
     kids started giggling.

     "Ok, we just need more pressure", I dropped the gun on the ground, ran into the garage and dialed the regulator
     up to 120 psi. We picked up the unscathed potato plug and re-inserted it into the barrel. "NOW this puppy is
     going to fly" I shouted....

     This time the hiss was more urgent, the harmonic a bit quicker and with a slow "wa-THOOoooo" the little spud
     cleared the barrel and continued skyward for maybe another 10 feet before falling to the ground. "I thought this
     thing was supposed to go 200 yards?" Bill asked. We reloaded and shot the core up a few more times, the young
     kids ran to try and catch it, but quickly we began to loose our audience. When most of the kids had gone I
     noticed Bill staring at the tube with that bent gaze that he has when he is trying to solve some totally insignificant
     problem. I know this look. He is hooked and there is no stopping him now.

     "Ya know" he said, [when he starts with "ya know", it usually means that he has some really far fetched idea up
     his sleeve] "if you want to try something dangerous, I could try to hold my hand over the end while we build up
     pressure behind the potato, then let go." Naturally, with the apparent lure of danger, we couldn't resist (and the
     fact that Bill had volunteered to place his hand over the barrel of the gun). Most of the kids had dispersed and we
     longed for some real action. We held the tube at each end, almost completely horizontal, Bill pressed the palm of
     his hand hard over the end of the barrel while standing to the side. "Ok, go." he said. I squeezed the trigger and
     the sound of air pressure building started instantly. A micro second later, with a grimaced face, Bill moved his
     hand from the barrel. "FOOOP!" The potato core flew from the end of the barrel with yet-unseen velocity, hit
     Bill's hand and broke into pieces. Bill dropped the end of the barrel and strode quickly across the yard raising his
     hand up and down with an even more grimaced look on his face, just like I remember my dad doing when he
     smacked his thumb with a hammer.

     I think I have some idea how Edison felt when he saw that glow of light from the first light bulb in his lab. This
     was a huge milestone.  [Oh, about Bill. His hand was Ok. It continued to sting for some time, he came in the
     house and put an ice cube on it. I got kind of red. "It felt like someone whacked me across the hand with a
     broomstick" he said the next day. "It's still a bit sore"]   Suffice to say if I had not seen that potato go (and the
     look on Bill's face) I would have never pursued the "real" gun any further. But I saw the potential for something
     enormous, and so did Bill. "So what exactly do you need to make the real gun?" Bill asked, still swinging his


     Bill is one of those people who always is busy, seldom seems to have time to do anything remotely fun, always
     has something "scheduled"; until something pops up like this. Then, time is a non-issue. This is a symptom of the
     advanced stages of CUY.

     I ran in and got the cutaway diagram that I printed from the web page and went about explaining to Bill how it 
     worked. Like me, it took a few minutes staring at the diagram, then the  comes on. "oOOoohhhhhh, I see!" he said.
     He volunteered his wheels to run down to the hardware store to buy the stuff. We always have the same dilemma 
     where we live. Home Depot is 25 minutes away and is guaranteed to have what you need. But there a few 
     "hometown" hardware stores within minutes who might have it. We opted the try Aubuchons in Hudson, a local, 
     friendly place that for some reason I remembered having a fairly extensive plumbing isle. The real gamble, we 
     thought, was finding the 3" PVC. I didn't remember every seeing any PVC at Aubuchons.

     We got there and headed straight for the plumbing isle, they had lots of stuff. We found the 3" x 1 1/2" reducer
     pretty quickly, but the "cleanout" thing stumped us. We spent a good 25 minutes there rifling through all sorts of
     adapters, boots, and threaded gizmos; fitting part on top of part. Eventually my "cheap" side kicked in and we
     decided that a removable plug cleanout was overkill and that a simple 3" cap, if placed correctly would do the
     job. As it turned out, we were wrong about that. Just as we went to ask an employee about the 3" pipe, I noticed
     a rack in the back near the loading dock with PVC pipe stored upright. We walked quickly over and picked up a
     10' piece. 

     We loaded it through the open hatch of Bill's GTI and headed to my house. On the way home, we discussed the
     big mystery and only unprocured part, the "diaphragm". The web page wasn't real clear on what this thing was
     made of. All it said was: "Cut a 3 3/16 diameter disk of 3/32 thick polypropylene or polyethylene. A cheap
     dishpan from Kmart may be a good source of diaphragm material."   What's a "dishpan"? A plate? A food
     storage container? That tray you put dishes on to dry? I had no idea.

     Once we got home, Bill went about cutting a 3 foot piece of the 3" pipe with a hacksaw. The perpendicularity of
     the cuts on that pipe were not critical. I sliced the end off the 1 1/2" piece, just next to the cap I had glued on for
     the earlier, anti-climactic experiment. The web page said that the perpendicularity of this cut was critical. For this
     I used my 10" miter saw. It cuts PVC very nicely and smoothly if you go slow. Then I ground the little stop-ridge
     out of the inside of the narrow end of bell adapter and glued it on the 3' piece of 3" PCV. "Sliding" the bell
     reducer over the 1 1/2" piece was much harder than it sounded, in spite of the fact that I thought I did a good job
     grinding out the ridge. It was a very snug fit. We ended up laying the whole thing on the floor, with the 3" piece
     butt up against the garage wall, and placed a wooden block on the snout end of the barrel and pounded it with
     heavy rubber mallet. We stopped when the end of the barrel was about 1/4" shy of being flush with the back end
     of the 3" pipe section. We thought this placement was going to be pretty critical, so we did not glue the bell
     adapter to the 1 /1/2" PVC at this time so we could adjust it later.

     "Ok, what are we using for a diaphragm?" Bill asked. I shrugged. We knew that it had to be fairly flexible, but that
     it must also contain an enormous amount of pressure behind the barrel end. Within an hour, my work bench
     was littered with all sorts of plastic circles; and the tupperware cabinet in the kitchen was now void of at least a
     half dozen lids, sacrificed in the interest of science. We cut and flexed and examined, and realized the first
     mistake in buying a cap for the end instead of the cleanout with unscrewable plug. "We have to get it right the
     first time" I said, "once it is glued, it is in there". "Hmmmmmm" Bill replied, "well, if we really have to, we can
     just slide the whole chamber off the end to get access. Remember we didn't glue it yet."

     Yeah right. "Just slide the whole chamber off" We had to pound the damn thing on there with a 3lb mallet, now
     we would have to pound it back off with some kind of special "Spud Gun tool #23456" fork, then pound it back
     on again. I'm not sure which diaphragm we eventually chose, it is insignificant as as it never worked. We drilled a
     small hole in the back of the cap and squirted air in using the blow gun, but it just hissed out of the barrel.
     Dismayed, Bill agreed that we had to get the cleanout. I tried to convince him to run to Home Depot with me, but
     his CUY had waned and he said he had to get home.

     After dinner, I told my wife that I needed to run to Home Depot. As expected, I got the "We've got company,
     what could you possibly need now, that stupid gun, NO WAY!" reply. But a few hours later, I got "permission"
     to run out after all the kids were in bed if they were still open. I called at around 9:30 and found out they were
     open until 11:00. I ran out and got a 3" cleanout.

     Upon my return, I placed the gun up on the bench with the back end under the 10" miter saw. I carefully brought
     the saw down just in front of the glued on cap, cutting the 3" pipe but stopping short of the barrel pipe inside. By
     rotating the gun, and cutting a few more times, I cut the 3" pipe all the way around just in front of the glued on
     cap and removed the cap. I glued the new cleanout on the end of the 3" pipe. Now I had the task of cutting a bit
     off the end of the barrel off so that it was positioned properly when the plug was installed. As I write this, it has
     been many weeks since this experimentation and I don't remember the exact sequence of events. But for some
     reason at this point, I had glued the narrow end of the bell adapter to the barrel and was now stuck "guessing" the
     proper offset. I took some rough measurements, including the thickness of the plug, and using a thin carbide disk
     on the Dremel tool, cut the barrel from the inside as the diameter of the cutting disk was only about 1". I knew
     that I needed the barrel end to be perfectly perpendicular and that it was going to be all but impossible to get it
     square. It was.

     I could look at the barrel end and see that it wasn't square. So I put worn down 3" disk on my air die grinder and
     inserted it into the cleanout opening. I was hoping to use the face, not the edge, of the disk so smooth and square
     the barrel end. This work OK, but now there was another problem. The barrel end was now too far inside the
     cleanout. I used a cheater pipe and a pipe wrench to get the plug in far enough to hold the diaphragm against the
     end of the barrel. Eventually, I ended up buying another cleanout, and cutting the old one off in the miter saw and
     cutting all the way through the barrel too, making it square.

     But back to the diaphragms. I still had no idea what the "right" diaphragm was, but I had a few more candidates
     lined up. But with the cleanout plug I could now swap out the diaphragms easily. A started with a thin one, cut out
     of a Folgers coffee can lid. I had the regulator set on about 40psi. With the gun lying on the garage floor, no
     potato in the barrel, I tried it. Pressure built up in the chamber, but the air just hissed back out of the hole in the
     cleanout plug when I removed the blow gun. I cranked up the air to around 60 psi and tried again..

     When I released the gun this time, there was very loud and fast "FOP!" and something hit the recyclable bin
     under the stairs. With serious force.

     This was the second big milestone (the first was Bill's hand getting whacked, if you don't remember). That sound
     of air suddenly decompressing out of the barrel was very encouraging. And it sounded dangerous. What had
     happened was the Folgers lid had collapsed into the end of the barrel and was expelled out. Thus I knew I needed
     a more rigid diaphragm. Much more rigid.


     One of my "proto" diaphragms was cut from the bottom of a hard plastic food storage container, was by far the
     stiffest material that I had cut and was my next choice given the demise of the flimsy coffee can lid. I examined
     the end of the barrel to insure it was square and smooth, it looked fine, and inserted the thicker diaphragm and
     installed the plug behind it. At this point I wasn't really sure how far in the plug should go. Should the diaphragm
     have some "play" between the plug and the barrel? Should it be snug or should it pressed to make it "bow" a bit?.
     I eventually concluded (and have demonstrated) that they should have about 1/32" of play. 

     But at this time it still didn't work. When I squirted the air in, the diaphragm made a good seal on the end of the
     barrel and I could hear pressure build in the chamber. But when I released the blow gun, the air just hissed back
     out of the hole. The diaphragm was not being forced back off the end of the barrel for some reason. I didn't know
     why. It was past midnight on a Saturday night, I quit for the night.

     The next day, after the usual Sunday morning rituals, was my mother-in-laws surprise 70th birthday party.
     Needless to say, the opportunities to mess with the gun in the middle of the day were not realized. But my mind,
     all day at that party, was trying to solve that mystery. It was drawing diagrams on an easel with color coded
     shaded areas showing pressure differentials. It was doing state diagrams at blow gun release t-minus-zero and
     doppler color flow to identify areas of turbulence in the chamber. The program never stopped running. Sometime
     after the 17th party guest took the stage to laud my MIL, the program stopped and returned a result.

      The seal that the outer, backside face of the diaphragm makes with the front face of the plug is just as important
                      as the seal that the front of the diaphragm makes with the back of the barrel.

     Funny how you can be so certain that you have solved something without even verifying it. But it HAD to be.
     The backward movement of the diaphragm to allow the air to escape up the barrel was dependent on complete
     depressurization of the tiny cavity in the plug, behind the diaphragm. If the diaphragm didn't seal well against the
     plug face, all of the air in the chamber would just leak past and out of the hole (that is what was happening). I had
     been focusing only on the diaphram-to-barrel seal. The plug was an injection molded part and was probably far
     from smooth. I couldn't wait to get home....


     As soon as I got out of the car, I grabbed the plug and looked at it. Yep, it had burrs and rises as you would
     expect any injection molded part to have. Later that night, after the kids were in bed, I returned to the garage. I
     placed a full sheet of sandpaper on the bench, grabbed the plug by the square on the back, and rubbed its face
     back and forth on the sandpaper until it was smooth. I did the same with the backside of the diaphragm. I quickly
     re-assembled it, laid it on the floor and grabbed the blow gun. This was gonna be it. I squirted the air in the back,
     the diaphragm "whined" bit, there was some hissing, then all noise stopped. I let go of the gun and yanked it back


     All I could think about was those guys standing in the desert watching the first atomic bomb being detonated. The
     awe, the magic,... the danger. I pumped it a few more times. Predictably, and instantly each time FOPP!, FOPP!.
     Yes, this was the third milestone. I still had the bucket with the potato cores that we had experimented with
     earlier. It was around 11:00 PM. I turned on the flood light in the front yard, shoved a spud core down the barrel
     and laid the gun on the floor with the business end of the barrel on a piece of 2x4 laying flat, pointing out of the
     garage door. Thus, the gun was inclined by 1.5" over is 7.5 foot length.

     Now my house sits 150ft. off the road and is almost completely surrounded by woods. The gun was aimed
     diagonally out of the garage door, pointing over the front yard which sloped down just a bit. At the yards edge, is
     a bark mulch covered area with some very tall mature trees, behind that a stone wall, then about 100 ft of woods
     before you hit the street. I aimed the gun at an oak tree about 1.5 feet in diameter and pumped the gun.

     FOPP! The little spud rocketed across the lawn, missed the tree, just cleared the stone wall and vanished into the
     woods with the sound of something tearing through the light foliage, ripping leaves as it went. Most stunning to
     me was that its trajectory was a perfect straight line as best I could tell, it didn't "fall" at all. Second most stunning
     to me was that I missed the tree by a good foot at only about a 60 ft range.   I reloaded and fired again. I missed
     the tree again. This time the spud hit the upper edge of the wall and appeared to splatter into a hundred

     Just then, my wife came out of the house into the garage. "Linda from next door is coming over, she wants to
     know if you could look at Kevin's bike, something is broken". "Put your shoes on and come out here. You've
     gotta see this!!" I said excitedly, ignoring what she had just said. Just then Linda came into the garage. I got a
     piece of paper and drew a quick sketch of the gun and explained how it worked (they didn't seem interested nor
     like they understood). I showed them the "core cutter" (they seemed a bit more interested, something that cuts
     vegetables was closer to home) and then loaded the gun. "Watch this." I said sharply, "I'm gonna try to hit that
     tree". FOPP!! This time the shot was a bit lower (although the gun was in the same position) but I missed the tree
     again. Instead, the little 'tater hit one of my landscape lights. It went out. I looked up. Linda, the next door
     neighbor had a stunned look on her face. My wife exclaimed "Great. You broke a light. Great."


     I had no more potatoes and it was dark. I was dying to "see" how far this thing would propel a veggie. I
     contemplated asking Linda if she had any Idaho's I could borrow and timing the interval between when I fired one
     straight up and when I heard it hit the ground. I could get out my old Physics book from school and figure out the
     height traveled. But I didn't, I went inside.  The next day after work, I went right to my son's soccer practice. I
     rushed home afterward in the last few minutes of daylight to fire a spud upward and see how high it would go.
     My wife had actually gone grocery shopping and bought a new bag of brown Idaho potatoes.

     I cut 4 cores and dropped them into a bucket of warm water. My philosophy was that the warm water would
     soften the outsides of them a bit and make for a better seal. I had no mount for the gun, so I brought a stepladder
     outside and propped the end of the chamber on one of the top steps. I pointed the gun as perfectly straight
     upward as I could, squeezed the blow gun and released fffFFFOOPPPP!. I couldn't believe what I saw. This little
     core went skyward faster than anything I had ever seen before. It violently wiggled around as the air stream
     worked on it. It continued upward for about 5 seconds, then started an accelerating plummet back to earth. It
     landed with a thud on the neighbor's roof.

     The "Backyard Ballistics" web page cites that this gun can propel a spud some 700+ yards. That is almost 1/2
     mile. I have since fired this gun at 120+ psi straight up, and while I don't know for sure, my estimate is that it
     travels straight upward at least 1/4 mi. It is absolutely mind boggling. Since my house is completely surrounded by
     very tall woods, I can't really fire it any distance and actually see where it lands. Yes, there is another
     neighborhood over the trees and up a hillside, the thought did occur to me. But since I would never see where it
     landed, I didn't see the point. One day I'll by one of those portable tanks and take the rig to the park.

     I have fired this gun at big boulder in the backyard at about a 30 yard range. I literally never saw the spud. I just
     heard a loud WHAP! on the rock, and there was a wet spot there. No sign of potato. Anywhere. And recently, I
     fired it at my wood pile at 120 psi. But I missed high, and it traveled into the deep woods with the sound of a
     huge bullet ripping through foliage. But firing it straight up on a clear blue sky sunny day is still awe inspiring.

     I know that I have not divulged much technical information here, refer to the "Construction" and "Theory of 
     Operation" pages for more detail. 

     If you are into to big boy toys, this gun is hard to resist. It is cheap and easy to build, and the way it works is
     fascinating to me. I was told that the diaphragm type release is used in truck air brakes as a fast dump valve. But
     let me say again that this gun has the potential to do some real damage. It is for responsible, intelligent people. If
     you make one, the first time you fire it, you will see what I mean. I know that that warning will make many
     people build one, but be careful. I really think this thing could KILL someone under the right circumstances. 

Home     Disclaimer     How it Started    Op Theory     Construction

Updated 21 Oct 2002