Er... no. Railguns can fire projectiles at speeds considerably faster than gauss guns, and all told they provide a bit more punch for the power requirement, but they're nowhere near relativistic. Think of it this way: People have build railguns. Hundreds, if not thousands, of technically-minded kids and physics classes. However, you'll notice a shocking absence of both stories about an entire residential block being blown up when a doped pellet traveling at 1/15c hit a wall and started a small nuclear reaction, and cautionary tales of entire classes being sterilized by the hard radiation of said pellet smashing apart air molecules that happened to get in the way.
Relativistic speeds are no joke, and if all you needed to take a sizeable chunk out of the planet would be a simple railgun the size of a semi and a decent power plant, have no doubt that somebody would have done so by now.
I think you were confused by the way the acceleration in railguns works. The EM force ramps up the barrel at roughly the speed of light, yes, but due to inertia, friction, et al. the object being thrown kinda lags and is pulled along behind it, and never even comes close to that speed. I suppose if you had a slug with a mass of 0, you could throw it at just about C, but then that wouldn't be much fun, would it? No kaboom, no nothin'.
But, ah, railguns are lame. They cook themselves after one shot, and for a single-use weapon they're pretty expensive and pretty puny. Stick to gauss guns- now THERE'S some cool ****.
Ever read The Moon is a Harsh Mistress? MDW-licious!
As to the topic of this thread: Yeah. Make an oblong "laser" bolt, give it a really high speed. If you want a particle trail, treat it like a missile. This spirally crap isn't even fantasy, it's just some stupid thing someone decided would make their gun look cooler.