This video will explain how I made this wrist gun. Sorry I couldn't go through everything step by step because I had already made it at the time all you need is the barrel (which I got from a handy gripper), a CO2 inflator that runs on cartridges, and a arm brace. I made my arm brace out of sheet metal and other metal parts but you can just purchase a arm brace online.
Links for the parts:
Wrist Brace: http://www.amazon.com/Medline-Universal-Wrist-Forearm-Splint/dp/B007PKSJFG/ref=sr_1_2?ie=UTF8&qid=1437321784&sr=8-2&keywords=arm+brace&pebp=1437321791818&perid=0SHFEKTHF358ERMQXW32
Barrel (The barrel came from a reacher grabber): http://www.amazon.com/Ergonomic-Lightweight-Reacher-Grabber-Rotating/dp/B000EWXSSK/ref=sr_1_4?ie=UTF8&qid=1437322048&sr=8-4&keywords=reacher+gripper&pebp=1437322052483&perid=0P0DJ6Y764Z78XKM56FT
Co2 Inflator: http://www.amazon.com/Genuine-Innovations-G2628-Ultraflate-Inflator/dp/B002AGI1QW/ref=sr_1_1?ie=UTF8&qid=1437322138&sr=8-1&keywords=co2+inflator&pebp=1437322142806&perid=16GBNZ8JEE3QFV1DEB6F