A vacuum core superconducting inductor is actually the best choice, but opening switches are an issue....(a new version of which I am in the process of patenting).
Do you mean
closing the switches? Or opening them as the missile passes given points in the fields?
I designed, but never built, a tiny one for throwing a 3/8" section of 1/4 inch aluminum dowel, using braided "solder wick" for contacts instead of rails. But my son borrowed my big caps and I haven't seen them since. I also intended to use the big neo magnets from a voice-coil driven hard drive for the mag field.
If I had got the thing to even eject the aluminum rod, I would have considered it a great triumph.
I was pretty sure I got the right-hand-left-hand motor and generator rules mixed up, as well as "north-seeking" versus "true north" poles of the magnet mixed up, but I figured there were only four combinations to try in order to get it right.
Might have shot backwards on one combination, though.
Still, a micro-triumph. Yay, Terry!
Terry, 230RN