PVC is some pretty versatile stuff, especially when it comes to homemade weapons. You can make a homemade paintball gun, a carrot-firing rifle, or even the Burrito Blaster spud gun. YouTuber Top City Gear (aka Drew) made a PVC blowgun a few months ago, but he wanted something that packed a little more punch, so he built a pneumatic Nerf gun that looks like it really hurts—watch him try it out on his friend in the video below (skip to about a minute in to see it in action).
To make it, Drew used a gutted cordless drill, a sprinkler valve, and, of course, some PVC. The reservoir fits into the drill, which is wired to the sprinkler valve to give him more control over his shots. The battery of the drill powers the trigger and opens the valve to release the pressure, which is what causes the gun to fire.
You can watch the video for build details (skip to 8:25 to see it put a dart right through a piece of wood) and check out Drew's YouTube channel to see more of his projects.