capacitors charge to the source voltage. if you want higher voltage on the caps, you need to step up the input voltage to charge them to a higher voltage.
and, I would not recommend using a mechanical switch instead of an SCR. due to the wonderful nature of the amount of power you'll be dealing with, the switch will probably be destroyed before long.
and don't expect much out of a single-stage coilgun either. ESPECIALLY if you are switching it mechanically. to be truly effective, you have to turn the coil OFF as soon as the projectile reaches the center of the coil. otherwise, with the coil still on, once the projectile passes through the coil, the coil will then be pulling it BACK, which will slow it down... it may still launch, but only go a few feet. to do the switching, you either need some timing circuitry, with a specific projectile and starting location, or you need a photo interrupter type circuit that will detect when the projectile reaches a certain point in the tube. Either one of those is impossible if you're simply switching the device manually.
My guess is that you don't quite realize the complexities of what you're getting into... nor the dangers of large capacitors and high currents being switched through coils... maybe when you first weld the contacts of your switch together you'll see :lol: