Also note that the back emf from the coil will destroy the capacitors so make sure it's either critically damped.
You need to ensure that the current as decayed when the projectile has been ejected. This isn't easy, you'll need to play around with the capacitor size and inductance of the coil to get it working properly.
I did have a spread sheet I downloaded but I can't remember where I got it from and it's on my work's computer so I'll get it to you tomorrow.
**broken link removed**