I have an arguement going with a friend of mine who says that the muzzle velocity of an electric rail gun has nothing to do with the length of the barrel. I say that the length allows for more pull, so that it is easier to get to a specific velocity. He says that only the current and the mass of the projectile matter.

We were theoretically talking about using a railgun to launch iron asteroids. I thought that 100 meters was too short for an object the size of bus, weighing 500 tons or more, to get to a velocity of 3,000 to 5,000 meters/second. He said that was crap and it would easily be able to get to that speed given a large enough current and a stable enough mounting.

He also said that theoretically they should be able to fire at relativistic speeds.

What do you think? Anyone know anything about railguns?

I agree with you. The current of the rail gun and mass of the projectile will control how fast the projectile accelerates inside the gun. A longer gun will allow the projectile to accelerate for a longer time, thus be moving faster when it exits the rail gun and stops accelerating.
On the other hand, a rail gun 100 meters long could accelerate a projectile weighing 500 tons to 5000 m/s if it had very powerful current indeed. The acceleration would be tremendous but it's not impossible.
As for relativistic speeds, again that's possible, barely, for a large object but the energy needed would be incredible. Plasma engines such as a few spacecraft have tested accelerate charge particles to very high speeds and are similar in principle to rail guns but the particles they accelerate weigh almost nothing. Accelerating an asteroid to relativistic speeds in a space of a few thousand meters probably isn't possible. The magnetic fields in the rail gun necessary would be so powerful that the rail gun itself wouldn't be able to support the loads.

I would agree with you, you need enough length of sequential magnets in order ot get the speed of the projectile up to speed...