To store the energy for the intense magnetic field, I used 17 disposable camera capacitors arranged in parallel. These capacitors are charged by four identical camera capacitor charger circuits whose outputs are wired in parallel to the capacitor bank. The inputs to the chargers are wired in parallel to a normal-duty switch, which connects the charger inputs to four AA batteries, also in parallel. All this parallel-ness makes the capacitors charge faster and lets them dump more power all at once into the coil.
The "trigger" for this coilgun is a heavy-duty (20 amp/250VAC) switch wired between the coil and the capacitor bank. While this setup works decently well, I have noticed that the switch contacts occasionally weld themselves together from the massive (possibly lethal) amounts of electricity being flung across them. I may fix this at a later date with a better solution.
To make the coil, I took ~90 feet of 30AWG wire from a microwave oven transformer secondary/high voltage winding. I cut this in half and twisted the ends together to make two strands in parallel. I then wound this around a pen tube with two acrylic end stops 30mm apart from each other. I did 10 layers this way before sealing the wires tightly in place with hot glue. The coil began 30mm from the frontmost part of the end cap of the pen.
The projectiles I am using are 30mm sections of 1/4-20 bolts and 30mm sections of 6mm steel rod. They work really well and are cheap. I placed some of them in a copper sulfate bath (after a brief cleaning dip in 25% H2SO4 and then in aqueous NaOH) and then polished them to give them a shiny copper plating, which adds to the "authentic bullet look."
As you can see from the video, it's a pretty fun toy! To wrap up the project, I did a test measuring the projectile drop in altitude versus the distance of the shot to calculate the muzzle velocity. As it turns out, this gun has a 12 m/s muzzle velocity with a 7g steel bullet. The bullet then has a kinetic energy of 0.5 joule. *Sigh* Ah - it's still fun even if it's weaker than a BB gun.