Who said anything about huge? Transformers are available in 1" cubes. And a 555 timer cicruit is nothing, even if you decide to go through-hole. A MOSFET could handle reasonable current with a small heatsink. Ever seen inside a PC's PSU? nothing major in there and they run at 600W+
If you want to charge a 960uF cap bank to 700-800V in seconds. Do the maths (I refuse to exclude the 's' ^^) , work out E, then you can work out what power you would need for a certain charge time. Of w're talking 20 seconds, then thats, what...er..12W? At 700V thats 18mA.
As someone has already pointed out, getting a large voltage from a boost convertor is tricky. Sure 12 -100V is simple enough...but 600? I'm sure theres plenty of info on the web about this. Not only for coil guns but exploding wires, lorentz cannons, rail guns...all require large joule cap storage for anything fun.
Blueteeth