Here is one of the projects from the book '15 Dangerously Mad Projects for the Evil Genius'. You can actually download the chapter in full, free of charge, from here so I will just give an overview of it here.
Its pretty simple to make and safe. I kept the voltage low (36V), although it does pack about 100A through the coil in 10mS and it will fire a small section of nail at a maximum of about 30 mph.
WARNING: BE SENSIBLE! DONT FIRE IT AT ANYONE, INCLUDING YOURSELF.
Here is the schematic.
And this is how it is wired together.
If you fancy having a go, be my guest and download the chapter and if you like it, then you might like to try some of the other projects. There is a real mixture. Some like the Trebuchet don't actually involve any electronics at all and three of them use Arduinos.
Chapter 01. Coil Gun
Chapter 02. Trebuchet
Chapter 03. Ping Pong Ball Minigun
Chapter 04. Mini Laser Turret
Chapter 05. Balloon-popping Laser Gun
Chapter 06. Laser sight
Chapter 07. Laser Intruder Alarm
Chapter 08. Persistence of Vision **** Arduino
Chapter 09. Covert Radio Bug
Chapter 10. Laser Voice Transmitter
Chapter 11. Flash Bomb
Chapter 12. LED Strobe
Chapter 13. Levitation Machine **** Arduino
Chapter 14. Light Seeking Microbot
Chapter 15. Surveillance Robot **** Arduino
There is also a video on youtube showing the more interesting projects.