This DIY pistol crossbow is very dangerous, so make sure you use extreme caution after construction. You don't want to DIY yourself a jail sentence!
So now that that's out of the way… watch and learn how to make an awesome pistol crossbow. You can find plans for this crossbow pistol and images of this crossbow at Mechanical Mashup. This is a very detailed homemade weapon, so if your woodworking skills aren't up to par, then maybe you should go back to shop class.
This pistol crossbow was designed by Edwin Love, featured in "Science and Mechanics" magazine, March, 1963.