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!