Bolt Action Pneumatic Spud Gun

No one should ever make a potato cannon. They are wildly unsafe, powerful, and can easily shatter your neighbor’s gaudy bay window, you know the neighbor with the mean dog.

That said, [Jeremy Cook] made a minature bolt action spud gun! Using a custom machined Delrin bolt, a fitted Delrin reducer, and some PVC, the spud gun is capable of quickly loading custom shaved potatoes with the greatest of ease. Pushing the bolt (literally a bolt) forward forces the spud through the reduction coupling and into the barrel. Lock the bolt to the side, pull the trigger (an air blower) and two expansion chambers toss compressed air behind your starchy projectile. The design is reminiscent of  a common bolt action rifle, but all that Delrin reminds us of paintball markers.

[Jeremy] is writing up the project in multiple posts, so check his blog for info. We are also no strangers to the strange, dangerous and wonderful world of potato launching tech.

Stick around for a video of the launcher in action!

Comments

  1. Mirlotus says:

    nice! :D

  2. macw says:

    that’s adorable. I’d love to see one where you could put in an entire potato and as you cycled the bolt it would drop down and slice properly-sized chunks out of it and load them into the barrel.

  3. addidis says:

    If I was going to recommend a spud gun project I would have to recommend combustion types. But 3 of them exploded during development its seriously unsafe and it would be dumb to do. That being said… When the fourth revision fired it looked like a howitzer. About a 6 foot streak of flame came out the business end. We /never/ found one single projectile fired from it. It was shot in a 3/4 mile field with another mile or so of field behind it. We discontinued firing it due to concern we were damaging property . This was the second most dangerous thing ive ever made.

  4. Greg says:

    Weak. Combustion with simple hairspray has much more kick than that. And not to mention Ive never had one of the 6 I’ve made explode on me. Polish your design enough and get the right air/combustion ratio and your set. Awesome bolt design, but the product shows little to no kick=weak

  5. bobbus77 says:

    This model looks well suited to carrots. Anyone ever tried making a carrot gun?

  6. therian says:

    if you afraid of shrapnel wrap you gun in thick cloth do it loosely so air have some space to escape

  7. silvesterstillalone says:

    @addidis. I hate to burst your fiery bubble, but I will. Not to say I don’t believe you; over a mile flights are hilarious fun.

    Compressed air based cannons are inherently safer, more reliable, and have been shown to be more powerful (in anecdotal evidence anyway). That is a talking point in itself with stipulations like using ‘normal’ aerosol fuels or starter fluid instead of, say, propane..nevermind the vast array of valves, the best of witch is a custom made piston exhaust valve by the way.

    Anyway if you are getting exterior burn then you are either using too much propellant or have too short of a barrel. It’s always the latter :). It is simply a waste to have overburn since it is not contributing to propulsion.

    Furthermore if you wanna go big and long (pun intended) without using explosives look into, and please don’t; it is crazy foolish, hybrid cannons. These are devices measure out a combustible gas in just the right amount to achieve best combustion using a couple ball valves and a second chamber calculated to the appropriate size. I forget what grill propane is, somewhere around 1:24 but it varies with additives and such. Oh but they are hybrids remember? You double, triple etc.. the amount of gas and then add compressed air in increments of ambient pressure. So lets say you have 1:12 (8%) of propane, then you fill to 29 PSI absolute. It’s rather shocking the power increase.

    Oh and since you are releasing compressed air and combusting at the same time it is best to use burst discs. Sides of @ liter bottles work well since they are rated at a minimum of 150psi. Usually we..er the people that do these things double or triple them up.

    Now no compressed air cannon can launch a projectile faster than the speed of sound since the speed of sound at temperature is the speed of compression, which is also the speed of decompression. Mathematically laid out it’s the ‘c’ of spudguns. As far as these things go hybrids are the only ones capable in practice of breaking the sound barrier.

    Note: I know these things in my pursuit to find out if a paintball can survive past the speed of sound. Answer: maybe. Don’t do any of this stuff, it is a really bad idea and illegal in many places.

  8. JeremyC says:

    @Jesse Congdon – Thanks for the write-up! Like the way you put the two pics together.

    @macw & mirlotus, glad you guys liked it! I’ve thought about ways to have some sort of “clip” for the cut potato, but havent’ gotten a good solution yet. A full size one that cuts whole potatoes would be pretty incredible…

    @addidis – As cool as they are, combustion guns really scare me. Not sure I want to know what the #1 most dangerous thing is.

  9. Wolf says:

    It’s a common misconception that combustion guns are less safe than pneumatics. In my experience, the opposite is true.

    As long as long as you’re not adding oxygen the chamber pressure doesn’t get much higher than 60 psi. Unless there’re huge cracks on the inside of the combustion chamber, there’s just no way it’s going to burst.

    Plus you don’t get that ear-ringing boom with pnumatics ;)

  10. CutThroughStuffGuy says:

    “No one should ever make a *PVC* potato cannon.”

    Fixed it.

    Air and PVC don’t mix.

  11. qwerty says:

    The mere thought that what is thrown away as a bullet here for fun is craved as food in some countries deeply offends me, both as a human being and a tech.

  12. madwelder says:

    There is PVC made for air service but it’s not common and I don’t know that it would handle shock loads like this.

    It’s dangerous and I’m pretty sure that potato guns are prohibited weapons where I live but having said that, when do we get to see a potato Gatling gun?

  13. Alex says:

    Me too qwerty. As a lover of fries and a descendant of potato famine survivors, this machine is offensive to me and should be removed from this site immediately. The people who made this machine should be imprisoned.

  14. therian says:

    @qwerty
    instead of bitching give yours proposition on projectile ? it must be heavy, lubricated and airtight. People didn’t randomly chose to waits

  15. Bill says:

    “No one should ever make a potato cannon. They are wildly unsafe, powerful,”

    and illegal in many US states, like Florida.

    “Owning or using spud guns in Florida is illegal pursuant to Florida Statute 790.161 Making, possessing, throwing, projecting, placing, or discharging any destructive device or attempt so to do, felony”

    I grew up building spud guns, so I obviously hate this.

    It’s a great build, just check your local laws before you go recreating it.

  16. Pedro says:

    I wonder how fibreglass wrapped copper pipe would perform under hydro test…

  17. madwelder says:

    I feel a stroll down amnesia lane coming on!

    In the village where I grew up there was a man with a ham radio and he would use a potato gun to launch wires over trees to make giant antennas.

    He was also friends with the owner of the boat I cooked on and once they stayed up all night to drink and launch potatoes out into the inlet. We headed out the next morning and after we were under way I found out they had used up all the potatoes!

  18. zerocool42 says:

    @therian – Who said anything had to be fired. Not to mention, I’m sure it is possible, albeit perhaps not economical to craft another suitable, non-food based projectile (after all, the alternative is that it’s IMPOSSIBLE to create a synthetic, no-edible potato!)

    However, that said @qwerty, those potatos, weren’t ripped out of the hands of starving people. They were probably bought at a supermarket, where, if they hadn’t been bought, they would have been thrown away. So arguably, the real issue is with the supermarkets. Furthermore, a donation to an appropriate charity would be more beneficial, and while the maker doesn’t say they did such a thing (and I think it is highly unlikely that they did), doesn’t mean that they didn’t. You can’t criticise unless you know!

    @Alex – I sincerely hope you are joking, or at least exaggerating. Suffice to say, there are worse things happening in the world today. If you were not however, then see what I wrote about supermarkets in the previous paragraph!

  19. JeremyC says:

    @All, I will turn myself in to the local prison immediately for wasting potatoes and constructing what is essentially the same thing as a nerf gun.

    @Bill – I’m originally from Florida, and if you are too, you may remember how Governor Lawton Chiles fired a potato cannon on his inauguration:

    http://articles.orlandosentinel.com/1995-08-14/news/9508130255_1_potato-launcher-spud-gun-potato-guns

    He was never charged with any crime.

  20. addidis says:

    @JeremyC

    I forget the name , but the most dangerous was the grey stuff on sparklers. It ignited as it was being made and burnt down my friends kitchen (this was all during 6-8th grade.

  21. Bill says:

    @JeremyC

    That was in 1995. Law wasn’t on the books till a few years ago. Of course he was never charged.

    Born in raised in Florida, still here today.

  22. allen says:

    if you are in an area that spud guns are illegal…how about a traction trebuchet?

    http://members.iinet.net.au/~rmine/Greycos/trebfil5.html

    I’m building one like this with modern materials using a front fork from a bicycle as the yoke and pulleys to get the rope-pullers back near the sling holder.

  23. nicco says:

    @JeremyC: Great build, kudos, and all that. One concern… please tell me that you aren’t using the bolt handle for retaining the projectile in forward battery. That is a bad idea. I know, it isn’t a “real” gun with a “real” bolt action, but there is a very real reason for using a locking lug on the bolt as a retention system. Your design, at the low pressures it works at, will probably not fail, but if it does, look at where your chamber is in relation to your body parts.

    @qwerty: How do you think we’re supposed to get the food bullets to the hungry? This is a food delivery device.

  24. graymalkin says:

    @addidis Did you consider that the spud may have just disintegrated? This is an issue often in pumpkin chunking too.

  25. hammy says:

    If you are concerned about the possibility of a PVC pressure-tank rupture, there’s an easy fix – make a forward-facing safety valve. *Approximate* yield strength for PVC is around 51.71Mpa (7500 PSI for yanks), so if you are using 2inch di piping for the pressure assembly (and assuming a FOS of around 2, you can have smaller if you’re feeling lucky), you will want a pressure valve set for roughly 288-300 PSI. gettting thicker-walled PVC pipe can kick this up to about 500 PSI. You can your tank smaller also to increase pressure carried if you do some thermofluid calcs to get an ideal tank/pressure combination – it really depends on your barrel length – more length means more gas at pressure required!

  26. hammy says:

    safety valve on the front of the tank, I mean. It is likely that how you connect it will actually be the cause of failure (the join or thread or whatever) rather than the valve itself so be careful!

    Making a point of failure on something that dousnt cause dammage (like on flexi-piping can often be a good thing to prevent big damage – similar to using shear-bolts or keyways when making pulley/gear systems.

  27. Alex says:

    PVC is not a good idea for air use.

  28. JeremyC says:

    @nicco – Thanks for the kudos.

    Yeah, the setup wouldn’t be all that great or safe for an actual explosion (and I’m not vouching for it’s safety currently. However, in this case, the barrel is only .6 inches in diameter, giving a cross sectional area of .28 square inches. I’ve yet to charge it past 80 PSI, giving a maximum force of around 23 lbs.

    @Bill – I did not know a law was passed in Florida about spud guns in ’95. That Chiles guy was pretty crazy though.

  29. JTaylor says:

    Most of the failures I have experienced were poor glue jobs or cheapo threads on caps(old hairspray type). I knew a (stupid)guy who used OxyAcetylene and damn near blew is arm off.

    Most failures are overpressure. Next up is stress fracturing.

    The heavy fabric thing works …up to a point. The better soultion(albeit, more expensive) is stronger materials in construction.

    To those complaining about the waste of food…go bitch at McD’s or other fast food joints that toss out tons of food. Really. IF you are that offended…go out and DO something about the situation rather than complain about how “evil” this is. Here’s a thought…find a way to improve it to your liking and share with the rest of us.

    Very nice build.

  30. lurker says:

    Props to JTAylor for offering some thoughtful inshight…

    …It’s called a potato cannon for one reason – The ideal projectile, for shits and giggles, has traditionally been the potato. Up until about 4 years ago (anyone else notice this one?), potatoes, in america, have been rediculously cheap as a food source and of superior quality… I eat a lot of potatoes, perhaps it’s the irish blood – so they’ve always been on hand – With proper barrel filing, a potato can be slapped on, rammed, and ready to fire in seconds… And a solid hit is just, for lack of a better word, spectacular…

    …Projectiles? USE YOUR IMAGINATION!!! A wad of leaves as wadding, and use anything for topping. Two in particular come to mind – Take a couple packs of firecrackers, fold them over themselves, and tape to fit the bore – Insert fuse-down, and you’ve got a kickasstacular airburst. Second, a wad of leaves topped with a fistful of steaknives made an absolutely terrifying, yet incredibly awesome spectacle.

    As for legality, if you live in the US, potato cannons are NFA (non-firearms) as per federal regulations. They are not guns, they are not destructive devices, they are not banned or “special” on any federal level. UNLESS your state has some caveat, you’re free to build whatever the hell you want. Do your research – The ATF has stated such several times in open and closed letters – As long as it’s muzzle-loading and you use a flint/piezo igniter, you’re more than likely fine.

    …Also, DON’T build anything that doesn’t have a “cleanout” style endcap – this is your last line of protection. Most pneumatic designs use normal caps, and if your system jams you’re screwed – The threaded endcap is the weakest point in the system and has saved my ass several times – Ever see a potato fly 200 yards and a cap fly 500? I have – and if I’d used a non-threaded system I can guarantee you I’d have the scars to prove it…

  31. strider_mt2k says:

    I can understand wanting to reduce food waste, but crying about being “deeply offended” on a hacking site isn’t really the way to get anything done about it if it moves you that deeply.

    Ultimately someone is always going to utilize some kind of something as a material to hack with that somebody else somewhere doesn’t have enough of.

    By that same logic, one could be equally offended over a water gun hack if one lived in an area with little drinkable water.

  32. localroger says:

    The reason PVC is not recommended for air isn’t that it can’t hold the pressure, it’s that if it does fail it tends to shatter into very sharp shards which can pierce flesh. And even if you stick to reasonable pressure limits, it can fail due to impacts, hidden fractures, and so on.

    Two safer alternatives I’d consider are (1) a used propane bottle (there are Web tutorials on converting them for use with pneumatic Holloween props) and (2) an empty canister water filter enclosure (because it’s thicker and the plastic won’t shatter if it fails).

  33. mic says:

    Wrapping pressurized parts in duct tape would reduce chances of shattering in a explosive fashion. But seriously everybody here owns a denim jacket and some safety goggles, right?

    Anyone talking about food waste? Millions of tonnes of potatoes and grain are left to rot because of quotas and market changes every year. It may feed a city for a year but if people can’t make money off it becomes garbage to industry. Problem, qwerty?

  34. Masta Squidge says:

    “I wonder how fibreglass wrapped copper pipe would perform under hydro test…”

    Probably fairly well. It wouldn’t be anything EXTREMELY high, however I own a 5000 PSI fiberglass wrapped aluminum air tank from a number of years ago. Its a VERY large tank, something like 113ci. Most people buy them for scenario games and pack mount them.

    But not me. No. I ran that thing in a massive drop forward and dealt with it like a man.

    But seriously. This thing needs an HPA source and regulator. This sort of thing is right out of the basics of airsmithing.

  35. willow says:

    *** For anyone who thinks tape wrapped around a chamber will lessen shrapnel:

    You’re a complete bafoon. I mean this constructively.

    A 2 or 3″ PVC piece at 100PSI will not likely fail. Failures are typically the result of the builder/user not properly factoring safety and/or failure to prevent the pressure from going over rated limits (see my comments below). So think more like 200psi, or 150+ on a 4″ piece.

    Figure a 2″ pipe 12″ long, and you’ve got almost 76″ of surface area with 200psi of hell behind it. That’s per-square-inch, or more appropriately, about 15,200 pounds of total potential overall.

    Go ahead and chance yourself with some duct-tape on that and let me know how it goes.

    I’ve witnessed failures first-hand because people over-pressurized chambers against everyone’s warnings. Like thieves and electricity, the pressure will flow through the least resistance and burst through a wall in a single spot.

    In a millisecond a few dozen (if you’re lucky) to a few hundred pieces of shrapnel will erupt from the thundering force, tearing through flesh, tendon and bone.

    Tape will help for about a split second, but the burst of air forcing laterally will rip the pieces from the tape. Sure, it might prevent a few pieces, but rest assured the force will be more direct rather than dispersed.

    *** For all the haters:

    When properly built, PVC is fine for air use. You have to make sure safety is your primary concern and follow important rules which are out there in the pneumatic/combustion community. I posted some general safety-violations below, which I found in this build.

    Been doing this for years, and the only failures I’ve ever come across from people are when they violate safety rules and don’t understand the scope of what they’re doing.

    Also, I keep track of my builds and know that age is a factor as well. You don’t want to keep using the same chamber over and over for years.

    *** For the builder and anyone prospecting:

    Kudos on the delrin bolt, nicely done and simple. The glue job looks proper and well done also.

    The pressure chambers (primary/secondary in this case) have a lower operating pressure rating than the barrel/action, which is important, so good job there also. This has to do with burst-stress/firing-stress, explained below. You never want a barrel/action which has the same or lesser rating.

    Also, the pressure rating stamped on the pipe isn’t your only factor involved. Those are nominal figures based on water at a specific temperature. Don’t forget that shock/stress has to be factored in, as well as temperature. I post a link below to some basic temperature de-rating calculations to help you along.

    *** However:

    Safety Violation: No PSI blowoff except for the tubing connecting the two chambers, from what I can tell. Burst-tubing is a good backup for overpressurization, but shouldn’t be your only safety net. Also that looks like 1/8″ tubing, which has a very high burst rating regardless of the material.

    Safety Violation: No pressure gauge. Sure, you can rely on the regulator for your primary fill source but that’s never an excuse to not know the pressure inside your chambers.

    Safety Violation: Threads tapped into single wall PVC (the sides of the chambers and on the endcaps), which increases the likelyhood of stress fractures on your primary wall of pressure.

    Threads should be tapped into double-walls instead, IE a sleeve glued over top of the chamber, or similar, so the threads are on the outer wall and a pinhole is merely drilled into the inner-wall (pressure-wall), lessening the likelyhood and severity of stress fractures. Stress fractures are the primary reason of pneumatic-PVC failures.

    Safety Violation: Threaded endcaps = surprise projectile somewhere down the road. Threaded connections are more likely to fail than their glued counterparts. This could result in a surprise projectile at some point. You shouldn’t be using threaded connections on pressure chambers.

    Additional notes: I’m guessing the primary chamber is 3″, which has the lowest operating pressure rating of all the piping. As such it should be what you base your highest operating pressure on. Most 3″ PVC has about the same rating, which is 150-160PSI.

    For safety reasons always take the lowest, so say 150. Now you also have to calculate your temperature derating figure. Go ahead and figure a factor of .62 based on 110F*, which leaves you with an operating pressure of 93PSI. That’s where you’d set your burst valve, if you’d like.

    For an additional safety net, some folks set their operating pressure to 75% of the calculated figure, which would be about 70psi in this case. WAY more than enough to have fun with the gun in this build.

    Here is a link to some basic derating factors: http://docs.google.com/gview?url=http://www.sovaspecops.com/forum/usrpics/3_8dcb84e01974857044dc5eb19e397986&chrome=true

    You always want your barrel to be stronger than the other points in the system, because of stress. Granted the projectile is easily moved down the barrel, so initial shock isn’t so much an issue, but for long-term use and the occasion object which might get stuck in the barrel, it’s better to ensure safety than to have shrapnel in your face.

    Keep the builds coming and keep them safe. Love seeing these things.

  36. willow says:

    Also, one last thing: Denim will not help you against the shrapnel unless you have a few layers. And most people don’t wear denim over their necks and faces and arms–the most commonly hit spots on the body in accidents related to these designs.

    Put your noggin to work a little bit more next time Mic.

  37. spacecowboy says:

    Nice workmanship. I never considered trying a machined Delrin bolt. As far as I know, many states define a destructive device as anything that uses combustion. Therefore you’re probably fine if you stick with the pneumatic guns–at least you are in Texas as the penal code expressly links destructive devices with combustion.

    Additionally, the speed of sound in air is not an absolute limit on the maximum velocity of a projectile of a pneumatic cannon–it’s not like C. As long as the projectile remains in the barrel, the expanding gas can continue to do work on it.

    The difference between potential and kinetic energy is expressed by the pressure difference. As long as the pressure difference exists, work can occur on the projectile. If work can occur, good times happen. Now if only this can scale to a 2″ projectile.

  38. marshmellowman says:

    I’ve used 1/2 PVC for miniature marshmallow launching using your lungs…. it works surprisingly well, but compressed air would be better, combustion may be tastier, but being hit with a flaming wad of melting sugar may not be that fun. Auto feeds with marshmallow may tend to jam. I wonder what a high velocity marshmallow would do.

    BTW, the first time I met my wife she came at me wielding a potato pellet gun… it was love at first sight…

  39. mic says:

    It is not a grenade idiot. Thanks for the sermon, willow.

  40. mginder says:

    replace that hose connecting your air supply to the barrel with pipe and fittings matching the size of the air tank. the rise in air throughput will multiply exponentially as diameter increases. the results will be much better than a skinny hose

  41. Matz05 says:

    I will call you “French Fry Rifle”! Muahahahaa!

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