How to Make a Zip Gun
To make a zip gun, you'll need a steel tube, wood board, solid pipe plug, 2 pipe couplings, 2 elastic bands, a metal strap, 2 screws, and a flat head nail. Start by drilling holes big enough to fit your ammo into the pipe and couplings. Next, drill a small hole in the center of your plug, push a nail through it, and file off the other end to secure your bullets in the loading chamber. After you've finished filing the nail, saw your wooden board into pieces to fit around the pipe and the stock. Once you've glued these together, screw your front coupling to the front of the gun and the rear coupling to the back. Then, screw in the pipe plug. To make a firing mechanism, bend a metal strap into a U-shape, drill holes in the ends, loop it over the gun barrel, and screw it to the stock. Finally, attach your elastic band to each side of the strap and stretch them to the front coupling.
Gather supplies. Conceivably, a zip gun can be constructed from household materials, but chances are you won't have everything you need. What's more, on top of the listed parts, you'll also need tools to properly file and drill the metal and wood pieces. These materials can be found at a hardware store, though there may be thing lying around your garage that will do the trick just as well. A receiver. A receiver is essentially the steel tube chassis for your zip gun. This is where all of the other parts will be made to fit in and where the bullets will rest. Ideally, it should be a 1/4' diameter tube to accommodate the most common types of ammo. Something along these lines can be found at a hardware store; improvised home gun makers have found success with gas and water pipe fitting. A 6 inch length for your pipe should suffice, although you may find it better to start with a longer length and saw it down to your preference. A wood board, 8'x5'x1'. This will comprise your gun stock. A 1/4' solid pipe plug and two steel pipe coupling, for the firing mechanism. Two elastic bands and a ~5 inch metal strap. Two #8 type screws and a flat head nail with a 1/16' diameter. Check the parts' integrity. Before going any further with making the zip gun, you should give each piece a check and make sure they are completely intact. If a pipe is dented or a coupling broken, the zip gun you make could prove very dangerous. Drill holes into the pipe and couplings. Drilling holes into metal will take precision and a high-quality drill. Drill a 9/16' diameter hole 3/8' deep into one of your couplings. From there, drill a hole 3/4' deep into the main pipe. Make the hole as big as you want to accommodate your chosen ammunitions; making it 25/64' (1 cm) in diameter will be enough to fit 9mm ammo. From there, fit the drilled coupling onto the pipe at the rear. If you have ammo at hand, you can use a bullet's size to gauge how big you need to make the hole. Make the cartridge plug. The pipe plug you should have acquired earlier should fit snugly inside your pipe. It will secure the bullets in the loading chamber. First, drill a hole in the center of the plug, just big enough to fit a nail through it. Stick the nail through and file it off at the other side. You'll be able to fit this plug in with the rear coupling when your zip gun is loaded. Furnish your zip gun's stock. A stock is important for any gun, as it'll give the gun far more stability in your hands and it'll give you something to build the firing mechanism on. With the wood you have, saw pieces to fit around your pipe, leaving space at the front for a pipe couple. Also make sure to saw off an extra board to glue on an the bottom so you'll have something to hold onto it with. Take care to leave an opening at the back for the pipe plug and rear coupling. Furnished properly, the gun should look boxlike, with the receiving pipe protruding at the end. Carpenter's glue is a good idea in this case to piece the wood together. You should give a few hours to let the stock set sufficiently. Attach the couplings. Once your stock has set to your liking, screw on the front coupling at the front of the zip gun. Following that, attach the rear coupling at the back of the pipe, and screw in the pipe plug. Prepare the firing mechanism. A makeshift firing mechanism can be tricky, but an elastic slingshot effect does the trick in the case of a zip gun. Take a strap of metal and bend it into a 'U' shape, drilling holes at the end to connect it with the rest of the gun. File a small notch on either side of the metal strap to fit elastic bands. Attach it to your wooden stock by putting two screws through the holes in the strip and securing them to the top of the stock. From there, attach an elastic band to each side of your strap and stretch them to the front coupling. Check for cracks and imperfections. If you have any intent on feeding your zip gun with live ammunition, you need to make absolutely sure that the gun has been constructed perfectly. This includes giving it a thorough look over to make sure there are no cracks in the material or flaws that occurred before or during the device's making. Load the zip gun. Loading a zip gun is fairly simple. Just remove the plug at the rear coupling, add the bullet firmly inside, and close it up again. Repeat this easy step whenever it comes time to shoot off another projectile. An improvised weapon can accommodate it own, invented brand of ammunition. However, this zip gun has been designed to work with 9mm ammunition, so chosen because it is a small calibre, cheap, and relatively easy to come across. Test fire the weapon. Firing the weapon for the first time can be stressful and risky. Due to the fact the firearm has been made at home, there is absolutely no assurance it will work safely. To save yourself the risk of injury, you should try activating the gun from afar. Go to an open space and attach a cord to the metal strap firing mechanism. Secure the gun in place so that you can control the firing direction. Find somewhere at least 3 meters away behind cover to activate it. Pull the cord and let the gun fire. If the loaded projectile fired successfully, you'll know you assembled the zip gun correctly. If for whatever reason the zip gun exploded, you'll have saved yourself a life-debilitating injury. Fire. You can fire your zip gun by pulling the metal strap back and releasing. Pull back the strap carefully and hold it with your thumb until you're ready to let it loose. Releasing the pressure will cause the gun to fire; make sure not to flinch when it goes off or you might send the bullet flying astray. Improvised firearms aren't made for range nor accuracy, so don't get your hopes up for hitting targets or whatever else. Zip guns are most effective at point blank range. Remove the shell. Once a shot has been fired, you need to remove the empty shell from the gun's chassis. Remove the shell by taking out the plug from the rear coupling and pushing it out with a thin rod. 
Gather supplies. Check the parts' integrity. Drill holes into the pipe and couplings. Make the cartridge plug. Furnish your zip gun's stock. Attach the couplings. Prepare the firing mechanism. Check for cracks and imperfections. Load the zip gun. Test fire the weapon. Fire. Remove the shell. 