Introduction: High Pressure Metal-Slug Cannon
There are plenty of PVC and ABS potato-gun guides on the internet, this guide aims to create something a little different. While it can fire potatoes (though they shatter), or anything that fits into a 1" barrel, it allows the use of extremely high air pressures to fire wooden, plastic, or metal slugs. Creating a metal cannon is more safe, far more capable, and as a consequence, more expensive.
While PVC can explode and shatter under high pressure (120< PSI), and ABS can split under medium pressure (70< PSI), copper can take far higher loads before failing. Rather than use combustion for power, this design uses compressed air. If the pressure goes too high, a copper weld will crack to release the air, rather than send out fragments like PVC. Copper also does not fatigue like PVC or ABS. The difference in strength between 110 and 150 PSI is immense, and 150->180 is even more extreme. Though like anything, the assembly has its limits, keep it under 350 PSI.
The cost ended up being around $100, and while the project is not very difficult, it is time consuming. If you feel the push to create an assembly of potential destruction, I encourage you to learn from, modify, and get to work on making your own copper slug cannon.
Slug Cannons and Potato Guns while very fun, have the potential to kill. We are dealing with a high-pressure design that is effectively a weapon. Use common sense, don't point it at people, cars, someone else's property, or anything else that can get you in trouble. With a wooden or metal slug, you really can punch through a car door, or break a person. By reading this you agree not to sue me for anything ever, including but not limited to any grief you cause yourself and others from the information in this guide. That said, have fun!
***Laws in California ***
These are the laws for this assembly in California, it's complicated, but legal.
Potato/Slug-Guns/Zip-Guns that use combustion, such as hairspray, propane, or butane, are illegal, since they are a non-licensed manufactured firearm. Using compressed air avoids this rule, and there is a second condition this assembly does not meet: fixed ammunition. A fixed round is a round that contains propellant, casing, and bullet all in one container. For this, there is no case, and the propellant is contained in the rear chamber, not on the slug, thus, it isn't a fixed round. Under ATF (Alcohol, Tobacco, and Firearms - Federal Law) it is legal.
Under California state law, there is a law which could be charged against you, called destructive device law. Any round over 50 caliber (which slugs definitely are) is a destructive device, if it is used with the intent to destroy something that would be an illegal act, like targeting a person or a car. Shooting a wooden target or dirt is not malicious intent. There is another technicality, smooth-bore (non-rifled) weapons that are muzzle loading (which this slug cannon is), are not destructive devices. This effectively puts it in the same legal category as an air-soft gun, despite them being very different (though air-soft guns are not muzzle loading).
Now realistically, when this fires at 150 PSI, the round whistles, a ton of water vapor pours out the barrel, and an echo is heard a half-mile away, which many people would report as a rifle or shotgun being fired. Most police will be very cautious, but likely tell you to stop and go do something else since they received complaints. Some police may confiscate the assembly or even arrest you, whereupon you go to holding, and will be let out after an hour or two and get your cannon back. In court, you will definitely be o.k. However, this is a huge hassle, and no one wants to go to a holding cell. Within a city, you will likely violate noise limitations or public-disturbance laws, which you can be fined for legitimately.
ATF laws [Title 18 United States Code (U.S.C.), Chapter 44, S 921(a)(3)] and [26 U.S.C., Chapter 53, S 5845] should be printed out. They define the classification of a firearm.
California state laws [CPC 12090-12094]; [CPC 12085-12086]; [CPC 12020] discuss the rules and classifications that relate to this assembly.
Simply put, it's legal in California, many cops think all potato guns are illegal since most are combustion, go fire it out in a rural area to avoid noise laws and attention. Print them all out, and keep them with you in case cops show up, and be friendly. CPC 12020 is the one that most police will be suspecting against you, and strongly emphasize that the assembly is compressed air.
Whew*, lots of information.
***End California Law***
Now let's get to building!
Step 1: Slug Cannon Parts
The total cost for everything in this project ended up being ~$100. The parts list is pretty simple, almost everything was purchased at The Home Depot.
(3x) 90 Degree 1" Street-Elbows. ($5.70/ea) ($17/total) [Street elbows are female on one end and male on the other]
(2x) 1" Regular Caps ($2.00/ea) ($4/total)
(1x) 1" Regular Tee. ($8.60/ea)
(1x) 1" Ball-Valve, Dual Female-Threaded. ($14.00)
(2x) 1" Regular Male Threaded Adapters. ($6.00/ea) ($12.00/total)
(1x) 1"x10' Red Copper Plumbing Pipe ($28)
**Copper Pipe Grades**
There are three grades that vary in thickness, from thinnest to thickest, they are: M, L, and K. L is good enough. Never go above 350PSI as [poorly done] solder joints can break, though really, 150PSI is very strong.
Oxyacetylene or Propane Torch (Propane Combo Pack for $18.00) - Oxy for brazing (above 800 F), propane for soldering (below 800 F). Heats up two metals to the point that solder can bond to them. I got a pack that contained the torch, solder, flux, and brush. You will also need a lighter, I use a modified BIC for an extra-high flame.
Hacksaw - It is capable of cutting many things, for this guide, copper pipe.
Measuring Instrument (Level/Tape Ruler) - This instrument can measure things.
Plumbing Solder - Effectively bonds two pieces of copper together airtight.
Plumbing Flux & Brush - Required for effective joining of copper pipes, when heated, it evaporates the metal clean.
Power Drill and bits up to 1/4 - Only for drilling a single hole into a cap.
Protection - Solder boils in this project, a splotch on your eye is something that should be avoided. I own some heavy-duty, chemical resistant gloves, which helped me turn the pipes while they were still hot (they don't compete with leather welder's gloves though). I also have chemical resistant eye-goggles, sunglasses are better than nothing. Wear a long-sleeve shirt and semi-thick pants (khakis or jeans).
Pipe Wrench or Crescent Wrench - For the final tightening of the assembly.
Bicycle Pump ($20-40) - Most bicycle pumps go to a max of 160 PSI, which is extremely powerful in this assembly. Get one made of metal, not plastic, as it could break if you really push it. There are bicycle pumps that max out at 240 PSI, but they can cost up to $100. Get whatever grade you want.
Cigarette-Lighter Air-Compressor ($~20).
This can plug into the cigarette lighter in your automobile, and pressurize the chamber without the physical effort and time of a bicycle pump.
Presta Bicycle-Valve Inlet (~$3) - I recommend getting a threaded Presta valve (all metal). Local bicycle shops tend to have plenty of destroyed tubes that they will give you for free from their trash. Allows a bicycle pump or cigarette-lighter air-compressor to charge the gun. This was obtained by going to my local neighborhood bicycle shop. You will also need two nuts that thread onto the valve, to attach to the cap.
Epoxy (Metal bonding) ($4 OR $15 for Thermal) - Used to connect seal the bicycle inlet to its cap. I recommend thermal epoxy normally used for computer processors if you can order it online, it is stronger, and can withstand blowtorch heat better than the cheap goop.
TFE Paste Tube ($2) - Makes the threads on the ball-valve air-tight.
Step 2: Measure Once, Cut Four Times - Pipe Design Lengths
The ten foot pipe will be cut into four sections. One 32 inches long, two 20 inches, and a fourth 12 inches. Mark each distance with a sharpie around the pipe, and cut away with a hacksaw, or pipe cutters if you have them available. It's easier to cut along the outside metal, rather than perpendicular within a cut.
It's difficult to explain the structure of how the pipes are laid out, please just view the diagrams I have created in the pictures below.
The Pipes are Termed:
20" (2x) Air Chambers
12" Barrel Connecting Chamber
Step 3: Sand, Flux, Torch, Braze.
How to Solder Copper Piping
For a stronger bond, and to remove any large surface impurities (dust, lime) you can scrape the pipe with sandpaper, or use a pipe brush.
Copper usually is not welded like steel or aluminum, it is rather joined with solder, or brazed with oxyacetylene. For this guide, I will only be using propane soldering. You brush on flux to the connecting surfaces, and put them together with a slight twist (1/4 turn). Lay the pipes level, that is, perpendicular to the ground, we want gravity to work for us. Heat is applied to both surfaces via a blowtorch which will cause the flux to evaporate and clean the metals.
You want the metals to be hot enough that solder will readily melt upon touching the metal. If smoke starts pouring out, the flux is burning, and you're using too much heat. Apply solder to the top of the joint, and the solder will wrap itself around and be drawn into the joint.
If the solder itself melts and just sits on top of the copper, the pipe is not hot enough. Heat the pipe until the solder will be attracted to and bond to it. You can rotate the pipes until the entire circumference has been joined, use either a thick rag or heavy-duty glove when gripping it. Once your solder joint is complete, set it on the ground and let it sit for 10-20 minutes, don't quench it by dipping it in oil or water as that will weaken the joint.
Pieces to Join
32" Barrel - Solder on a single 1" Male threaded adapter to one end of the barrel.
20" Air Chambers - Solder on one 90 degree Street-Elbow joint to each pipe. For one of the chambers, solder a cap onto its base, we will tend to the second cap in the next step as it is our air-inlet.
12" Barrel Connecting Chamber - On one end solder a 1" Male threaded adapter, on the other, a 90 degree Street-Elbow joint.
Step 4: Air-Inlet Valve
This step is the hardest part of the install, attaching our Presta valve inlet to a cap. There are two copper caps which will attach to the rear arms of the assembly, one of which is going to become the point where we can inject air via bicycle-pump or cigarette-lighter powered air-compressor.
Drilling with Power
Start with a small 1/16" drill bit, drill a small hole in the center of your cap. Slowly progress up in bit sizes to 1/4". At 1/4", the Presta valve will not fit, so grind away with the side of the drill bit until it does. We want as tight an opening as possible, ideally, it should require some real monkey-fisting to push the valve through.
Valve Attachment: Threaded Valve and Nut (Primary)
For making the seal, obtain a threaded Presta valve, and two nuts (which can require a second spare tube). Cut off at much of the rubber as you can, and apply the first nut about 2/3rds down to the base of the valve. Push the valve through the cap, and thread down the second nut, but not all the way. Carefully apply the thermal epoxy to the copper underneath each nut, then tighten them both down with pliers. Finally, apply epoxy on top of both nuts, and let it cure for thirty minutes to an hour.
Step 5: Assembling the Rig - Valve, Air-Chambers, and Tee
Connecting the Valve
This part requires a bit more precision, soldering our inlet-cap to its 20" chamber. The epoxy is not designed to take temperatures over 200 F, however we are heating the copper to around 600 F. Apply most of your heat at the juncture where the cap meets the pipe. Just try to minimize excess heating near the cap-end. Let the pipe cool for 10-15 minutes before commencing the next test.
Simple Pressure Test
Connect a bicycle pump to the valve inlet, place your hand over the 90 degree elbow, and pump a few times to make sure air comes out. This is to make sure the valve-inlet has not been sealed by melting epoxy or rubber. When actually pumping up the chamber to 160 PSI for loading a round, it takes about 20-25 seconds of pumping.
Soldering the 20" Air Chambers into Tee
Pretty simple, the elbows of the two 20" chambers go into the sides of the tee. Ensure that they are level with one another. On both elbows and tees, there are lines denoting their edges.
Soldering the 12" Barrel Connecting Chamber
There are two options for which direction the barrel faces, I prefer to make the legs face away from the barrel, so I can either slot my arm into the split, or rest the gun on my hip. Whatever you choose, just be sure to pick a direction you will be happy with. Solder the elbow of your 12" barrel connecting chamber into the top of the tee, try to make it level, as with its lateral force it likes to sag.
Step 6: Ball Valve and Barrel Mounting
We are getting very close to being done!
The valve can go on in two directions, while the firing lever only turns one way. Assemble your cannon (not tight, just for holding) to decide which direction you would like to hit the lever when firing. If you look down the barrel and see a reflection, the valve is closed. If you look down the barrel and see pure black, the valve is open. Just imagine how you would prefer to hold it before we fully mount the barrel. For my cannon, the lever is level with the air chambers, "under" the gun, facing then body when firing. It can be fired by either the left or right hand; a push fires the valve.
It is time to connect the barrel and 1" Female threaded Ball-valve to our air-chamber assembly. However, simply using a great deal of force to tighten on the valve is not air-tight. If we were to pressure it up now, we could still hit 90PSI, but the pressure would rapidly leak out. We must use TFE paste to seal it.
TFE Paste is a stiff, slowly-hardening, lubricated putty. It will never permanently bond, so you can always remove the valve if necessary. You smear it into the threads with your thumb, and then screw on the valve. Paste need only be applied to the air-chamber end of the valve, as the barrel will not leak much pressure in the short time it fires out the projectile.
The valve needs to be tight enough that when you hit the firing lever it doesn't loosen, but not so tight that the lever is not in the comfortable firing position you chose earlier. The barrel can only tighten so much before it stops right?
The sealant provides a solution for this, since it has lubricants inside of it, with the use of a pipe-wrench we can have some wiggle-room in turning the ball-valve. With a pipe wrench, the lever can still be in a position we want, but not at absolute maximum tightness, it's o.k. to be semi-loose, but the valve should be tight enough that you can't turn it easily by hand.
Step 7: Optional - Taping and Secondary Barrel
If you would like your cannon to look more aesthetically pleasing, I suggest wrapping the majority of the rig in black athletic tape, spare the last eight inches of the barrel though, since potato guts will be cut upon it. Don't use electrical or duct tape, as they eventually become a gooey mess after a few months. Black athletic tape is advised, since it can get dirty and still look nice, as well as be drawn on with silver-sharpies. Normally for hockey-sticks, I got two rolls for $7 from The Sports Authority.
Why not give your rig a name, menacing or beautiful for it's destructive power? Mine has been graced with the title: The Monglerator.
Since the barrel connects to the ball-valve via threads, we can connect any size barrel we choose as long as it has a 1" male-threaded adapter at one end. I purchased a 2 1/2" x 2' copper tube, as well as a 1"->2 1/2" reducer coupling. Together they cost $50, which is fairly high. Pretty much any new pipe, aluminum, copper, or steel, is going to be expensive at 2 1/2" (This is why scrapyards are great!). Still, if you're want to fire larger potatoes or tennis-balls at high PSI, read below.
Both the adapter and reducer coupling are the same size, since they are both meant to go over a piece of 1" copper pipe. We have to cut a short, 3 1/2" long piece of 1" diameter pipe to connect the two of them. So solder on the adapter and reducer to the same piece of pipe, and after they cool, solder on your 2 1/2" barrel to the reducer coupling. Then screw in your new, alternate barrel to the ball-valve, and give it a test. Higher PSI is recommended, since you are firing a larger projectile, I suggest 160-350PSI.
Step 8: Ammunition Variants
You have a cannon, and with 150-300PSI of power behind it, the ability to obliterate many things. Your choice of ammunition is limited only by your imagination or what you can cram into a 1" barrel, here are my favorites.
The Common Rounds
Small Red Potatoes - for a 1" barrel.
Large White Potatoes - for a 2 1/2" barrel.
Potatoes tend to shatter when you go over 100 PSI.
The ultimate round, is one from solid steel. Online, it costs about $25 a foot per 1" diameter steel rod. I went to a metal scrapyard outside of town, and purchased a six foot long, 27 pound rod for $12, one that produced many, many slugs. I cut it with a mitre saw using a steel grinding disc. They are by far, the most destructive ammunition I have.
It's best to take your barrel with you, to make sure the steel bar fits. If you barrel is exactly 1" wide, and your slugs are too, then they won't fit. The slug should be just a little bit thinner than the barrel. My fit was so tight, that after grounding off the paint, they fit perfectly. You can use a mitre saw grinding disc, and just roll the slugs sideways into the lowered disc to grind away the paint and some metal if you wish.
These are where things get fun, since you can use any material you please, just cut it yourself. Copper pipes are rated by the inner-diameter, that is, a 1" copper pipe is 1" diameter on the inside, and thicker on the outside. With a slug, you simply purchase a rod of any diameter material that matches your barrel, and cut it yourself. Two to three inches long is the ideal slug length.
Home Depot has a variety of wooden rods you can purchase, of varying densities of wood. I suggest a hardwood like oak or maple, light woods such as pine tend to shatter or fracture on impact. Dowel rods are very cheap, I got a 36" Oak Dowel for $4, which makes 18 slugs.
Simple list of wood strengths that are common dowel rods, higher is better.
The Wikipedia Janka Hardness List:
White Oak: 1360
Yellow Birch: 1260
Black Walnut: 1010
White Pine (Shatters): 420
Poplar (Shatters): 300
I purchased oak and poplar dowels, oak for durable firing, poplar to watch them shatter.
You can cut them with a hacksaw or wood-saw, but I strongly suggest using a Miter Power Saw. If you have access to one (ask family or neighbors), you can cut up 30 dowel slugs in about 5 minutes.
I strongly recommend just purchasing a few entire 36" oak dowels (not cut) as ammunition, the full-size works very well.
Hot Glue Steel Couplings
1/2" steel threaded couplings.
You can find them in the electrical pipe section at Lowes, for $1.28 per coupling. Though they are made for 1/2" pipes, the metal walls are so thick that it is almost exactly 1" diameter. Hot-glue sticks are cheap, and by filling the round, we make it heavier, more durable, and able to catch air. This adds extra weight (force), and you would be surprised at how indestructible hot glue can be. To fill the slugs, simply place a coupling vertical on cement or concrete, and start pouring in hot glue. Let it dry, and you will have a hard, flat, front surface. The threads in the coupling ensure that the glue is impossible to tear out.
You wouldn't want to lose these like you would a potato or poplar slug, so I suggest spray-painting them fluorescent orange to find them.
If you don't want to take the time to cut your own dowels, or fill up couplings with hot glue, you can buy pre-manufactured rounds called C Batteries. It just so happens that most C batteries are a little under 1" diameter, and make for solid, heavy rounds.
Step 9: Completion - Enjoy!
Well, you've completed your assembly, and now it's up to you to decide where to go and what to shoot. It's a great deal of fun to shoot at a few 1/2" plywood targets, coffee cans, or simply a long-range steel plate target. There are many more forms of ammunition I'm sure you can think of, only limited by your imagination and what you can fit into the barrel.
I hope you have enjoyed this guide, if you expand or modify my designs, please show me pictures, I would love to see them. If you have any questions, praise, constructive criticism, or ideas, feel free to leave a comment.
Good luck, share knowledge, and enjoy.
As an optional step, which is beyond the scope of this guide, I recommend developing a spring-loaded action to open your barrel faster, and with less bounce/recoil than a human hand would produce. By using a 4", 20lb/force spring the lever allows more air to hit the round much more quickly. It is especially important when you reach higher pressures (Above 120PSI). Building one can increase the amount of delivered force by over 200% (guesstimated). Also recommended is spraying Lithium Grease on your ball valve to make it turn more rapidly, as well as lubricating the barrel. I have attached two photos and a diagram to build your own spring-lever ball valve from.
Also, it's really fun to add a green laser pointer to the barrel.