A rail gun is a purely electrical gun that accelerates a conductive projectile along a pair of metal rails using the same principles as the homopolar motor.
Rail guns use two sliding or rolling contacts that permit a large electric current to pass through the projectile. This current interacts with the strong magnetic fields generated by the rails and this accelerates the projectile.
Much more information about real life rail guns can be read on Wikipedia.org, here: .
The railguns have been utilized on a few super-weapons in Ace Combat, and even one fighter jet called the CFA-44 Nosferatu wields a downsized version of an electromagnetic weapon.