I came up with my own design so I'm not going to tell you how to do yours. I only ask that you cover any exposed wires to prevent electrical shock. The voltage is high enough to Hurt you badly.
Step 1: Materials
400v capacitors (I used 820uf)
Coil of magnet wire
Step 2: Charging Circuit
I tried many different circuits to try and get this to work but the only one that did work was one that I made. I tried using flyswatter circuits and small boost converters and camera flash circuits which are ok for small coil guns but the bigger ones need better chargers. I found a boost converter design on this website http://uzzors2k.4hv.org/index.php?page=450vboostconverter that is relatively easy to build and has a list of components to buy at your preferred electronics website.
Step 3: Caps and Wiring
Link the capacitors In parallel and hook them up to the books converter. Take the positive of both caps and connect it to one side of the coil. Take the negative and run it through a thyristor circuit with the trigger releasing 5v to the thyristor to allow the current to flow to the other side of the coil therefore making the circuit ready for a pulse of 400v.
Step 4: Last Minute Touch Ups
You now need to link the charging circuit to the 12v. That will charge the caps. I've added a switch to my circuit to control the charger. Once all the wires have been connected, you are ready. Firing is simple, finish charging, turn off charger, put in a projectile, aim, and pull the trigger.
feel free to check out some of my other instructables, and my vids on youtube:)