Have they fixed the problem where the gun destroys itself after three shots? (Edit: downvoters: this is a serious question. This is a (historical?) known problem with railguns.[0])
Bludino notes [0] a Popular Mechanics article that quote ls 3MA, with a plan to go to 6MA!
The way a rail gun works the projectile(or its casing) must make physical and solid contact with the barrel. This is because the electric current actually flows through the projectile from one rail to the other rail[1]. This produces huge amounts of wear on the barrel, making it extremely difficult to maintain the barrel. Both in form and in being able have the projectiles make solid contact.
[1] http://upload.wikimedia.org/wikipedia/commons/9/9a/Railgun-1...
Off the top of my head this would pose a few issues.
1) While the magnetic force must go from one rail to the other, the amount of electric force is effected by how much of the body is doing the conducting(if my memory servers me). So while you could use wheels, you would still want the current to flow through the entire(or large portion) of the projectile housing(for lack of a better term). This leads to the second issue.
2) Melting. With these huge amperage's there is also huge heat levels. If you were to conduct this much current through a small point(as would be the case with a wheel) the heat generated would be even worse. Most likely causing the wheels/barrings to melt.
Neither of which cant be accounted for, just issues that would have to be considered.
Even better would be a highly conductive grease non-flammable grease.
1) Rotating bearings still have internal friction, which will still generate tremendous heat at these velocities.
2) A lot of the heat being generated here is caused by the flow of electrical current, which isn't negated by bearings. In fact, it would probably be made worse, because any spherical contact patch is going to be tiny compared to a flat surface. The suggestion of using many small bearings would seem to alleviate this problem, but the smaller you make the bearing, the more fragile it becomes.
At its heart, this is a materials science problem. You need a material that can conduct the electricity, withstand the heat, and not break down under high friction loads while doing both of these. Simply dropping in a bearing doesn't solve any of these problems
The highly conductive, non-flammable grease is just another materials challenge. At the kinds of temperatures present in a rail gun, "non-flammable" becomes a very high bar for a lubricating fluid.