
S-99, do you know any relativistic dynamics? You could get a railgun up to 99% lightspeed, but the Lorentz dilation of its mass would mean it was...very...impractical. We're talking
really impractical. I can walk you through the math if you want.
And it would be very stupid in atmosphere.
Today, it's not electrical power that's the problem for railguns, it's the rails themselves. There isn't a material that won't be wrecked by the stress involved.
Other problems too.
EDIT: boosting a railgun round to 99% lightspeed involves a seven-fold increase in the projectile's mass and a corresponding sevenfold increase in power required (over that predicted by Newtonian dynamics). Assuming it's a linear relationship between power and mass.
99.9% is a 22-fold mass increase. It just gets worse from there on up.