In electrical motors,coil segments are placed in a doughnut shape (stator). A metal cylinder on a shaft is then placed inside. By causing a magnetic field to rotate in the stator, the rotor is attracted to it causing it to turn.
Instead of a doughnut shape, segments are placed in a spiral shape forming stator helix in the shape of a gun barrel and a projectile is placed inside. As the magnetic field rotates it causes the projectile to follow an upward circular pattern until it reahes the end of the stator helix reaching maximum velocity and flight stability as it travels towards the target.

A concept closer to that currently under development might be to use a hollow core projectile to complete a circuit between an inner shaft and outer gun barrel creating a mortar version of a rail gun.
The explosive charge placed between the inner and outer walls of the projectile would run its length. to fire, the projectile would be slipped over and down the center shaft using the projectile's hollow center. At the bottom of the bore, the projectile would be in contact with both the center shaft and the inner wall of the gun barrel. Power is then applied.
After the projectile exits the barrel, a spring loaded door or similar device closes the front opening of the projectile to improve flight characteristics since the projectile would not be spin stabilized due to the short travel distance as is common in more conventional mortar designs.