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Gather materials. Superglue the spring to the inside of the PVC end cap. Superglue a plastic disc to the other end of the spring. Drill two holes in the PVC tube. Use the PVC glue to glue the end cap onto the tube. Attach the launcher to your favorite Nerf gun with velcro. Prime the launcher. Load the launcher. Fire the launcher.
This design calls for a one-foot section of two-inch diameter PVC tubing, an end cap for the two-inch PVC, PVC glue, superglue, velcro, a spring, plastic discs, a paint can opener, a drill, a ruler, and plenty of ammunition. The best springs for this design are long, medium coil tension springs such as those found on the inside of the "snakes" in practical joke canisters that spring out when opened. Superglue this spring directly to the center of the PVC end cap. You can easily cut this plastic disc out of something household such as an empty laundry detergent bottle. Most importantly, cut the disc with a 1.5" diameter so that it can easily fit flat down the PVC tube. Center this disc on the opposite end of the spring and attach it with more superglue. You want to test how deep you can coil the spring into the tube before drilling the first hole, so fit the end cap onto the tube without gluing it and push the plastic disc all the way down with a ruler. Use that to gauge how far the spring coils down the tube, and drill the first hole in the tube one-half inch more shallow than the depressed disc. Drill the second hole at the opposite end of the tube about one inch from the end. each hole perforate both sides of the tube in a perfectly straight line. Use a drill bit large enough so that you can insert the paint can opener through the holes. Once you have your holes drilled, you can glue the end cap holding the spring into place on the tube. You'll want to give the glue thirty minutes to dry before testing the launcher. To make this launcher resemble an M203 grenade launcher, you can attach it under the barrel of a carbine-style Nerf or Airsoft gun with velcro. Simply peel the plastic backing off a velcro strip and place one end on the tube and the other on the gun. You can use the wall-safe velcro types, which will even allow you to remove the velcro easily when you want. For select Nerf models such as the Recon, you can use the sight that attaches to the gun and slides off instead. Attach this piece to the tube, and you can clip the launcher to the gun where that slides and locks into place. You can just as easily use this design without it being attached to anything else as well. Push the spring all the way down, then insert the painter can opener key into the hole drilled into the bottom of the tube. This will hold the spring coiled while you load the launcher. This design can hold a ton of Nerf darts. Insert the darts in sets of five or six to keep them straight as they drop down the tube. You can place about three of these layers to fill the tube. Once you have loaded the tube, place a second plastic disc with the same diameter as the first one you cut into the tube on top of the ammo load. Hold this disc down while you remove the paint can opener from the bottom set of holes and insert it back into the top set of holes in the top of the barrel. If you still have some of the old Nerf ball-type ammunition, these fit in the tube and fire effectively as well. Now you simply have to aim the launcher and pull the paint can opener back out as though it were a firing pin. The high load of ammunition may not explode on impact (obviously), but the spread of the ammunition gives you a much more effective idea of a blast radius. Just like an M203, the angle of the launcher is how you aim. Practice firing it several times to get a feel for the trajectory of the launch, and then you will be able to aim proficiently in no time.