Actually, for a railgun, you don't want highly conductive. You want it conductive, but highly resistive, so that the current pushes it along the rails. Highly conductive materials would simply create a circuit and drain the power out of the capacitors for the rail gun without affecting the projectiles. As for gauss guns, I'm thinking maybe you mean a coilgun? Don't need to conduct anything with that. The projectile in a coilgun never touches the coils. It's moved by pulsing the coils in a carefully timed sequence to move the projectile down the barrel of the weapon at an ever accelerating speed, and timing a reversal of polarity on each coil to then push the projectile as it passes and goes into the area of influence of the next coil. Basically, you shoot magnets, it's like a maglev train.