The coils are inductors, and so cannot be turned off instantaneously - you get a voltage across them proportional to how quickly you're changing the current. As

The coils are inductors, and so cannot be turned off instantaneously - you get a voltage across them proportional to how quickly you're changing the current. As you build a faster coilgun, it gets harder to deal with the voltages generated by the sharper current pulses necessary to do useful work. And presumably you're also increasing the current involved to get higher velocity, which also contributes to the engineering difficulties involved in managing the coil voltages.