well, if use all solid-state hardware, the firing of the second and third stages should be fairly easy- it depends on the length of the projectile. for a 3/4 inch projectile, i would say place the break beam eyes about 3/4 inch behind the coil. that way the projectile is close enough to the coil for the millisecond delay in the firing, it can be halfway inside the coil when it fires.... also, making a moveable second coil would be stellar- and getting a projectile radar gun (like at the paintball range) would be good for an instant result of increase/decrease in velocity. that way, we can see how far before close it too close, far is too far, etc. im making this as a science project with a 6th grade step brother of mine, he's one of those kids who love weapons so i figure this might show up his spring airsoft pistol, haha. the first stage i know i can accomplish- i've already harvested the capacitors out of about sixty cameras- although i found caps online that would be much cleaner/less wasted space, being as how disposable camera flash circuits take forever to charge, im thinking of ditching the camera circuits and designing a beefy charging system that'd charge it in a few seconds. also started looking at getting serious capacitors, i see a few 200 joule caps for about $35 each. im wondering if i should keep it cheap the first go-around- i've designed tmy first gun to use 24 120uF 330V caps, charged through six charging circuits so it doesnt take ten minutes to charge all 24 using one or two camera chargers. i think i'll start to build them both simultaneously- the beginner, single stage 24 cap version for him, and the multi-stage using beefy charge circuit and heavy duty caps. heres a color coded diagram i MSPaint'd up so that he could follow along with how it was all wired.