There's a big difference between a coil gun (gauss gun) and a railgun.
Gauss guns use external coils aligned down the length of barrel to launch a projectile that can be magnetized. Basically, you have a row of discreet electromagnets. You turn on the first one to start the projectile moving. As it gets close enough, the first electromagnet turns off and the second electromagnet turns on (pulling the projectile). Repeat as necessary. A simple mass driver, if you will.
A railgun has two metal rails (in parallel), connected to a power supply. The projectile is conductive and completes the circuit. The current going through the now completed circuit makes the railgun into an electromagnet, and makes the projectile move down the rails (via Lorentz force) away from the power supply.
Now, there is a way to combine these two into a weapon. Basically, wrap a helical barrel around the two rails of a railgun. Hrm. I wonder if I could get some of that green "hybrid research" grants. Combining a railgun with a mass drive just has all kinds of interesting potential.
