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
(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.