A Mechanical Engineering student at MIT has created his own grappling hook launcher out of various materials, including an N-Strike Raider Rapid Fire CS 35 Nerf Gun, a dry chemical fire extinguisher and paintball coil hose:
Although I call it a grappling hook launcher, it is basically a souped up version of any pneumatic gun. I took the best aspects of many different spud guns to make one of the best possible systems doing my best to avert creating any weak spots and not using any dangerous PVC (which shatters when over pressured and does not work well in the cold). This system is designed to be robust and efficient in every aspect of its construction. It is not hard to build and the most trouble anyone would have is gathering all the materials needed.
Check out the video after the break to see the final product in action.
Step by step instructions on how to build one yourself, as well as a complete materials list is available from Instructables.