[scorpion_tyr]

Railguns achieve their damage by the high velocity of the projectile. Dialing down the power to subsonic wouldn't make sense at all. You might as well throw the projectile. Throwing a suppressor on it would not work at all against the supersonic "crack". The only way to silence that would be to design a projectile that could achieve such high velocities without creating the sonic boom. Look into some of the research done by Skunkworks. Much of it will be classified but supposedly they've achieved similar results with supersonic aircraft. All of this of coarse is hypothetical. Even if railguns become a common weapon, I kinda like the smell of gunpowder in the morning.