Two rails are surrounded by a helical barrel and the projectile or re-usable carrier is cylindrical.
A helical railgun was built at MIT in 1980 and was powered by several banks of, for the time, large capacitors (approximately 4 farads).
[2][failed verification] In 2007, a solid-projectile helical-coil electromagnetic launcher was successfully built and tested at the University of Missouri-Columbia (MU).
The device was powered by a capacitive-circuit which stored up to 125 kJ in energy, and achieved a 35.1 m/s muzzle velocity with a 145 g projectile.
[3] Previously MU had demonstrated a helical coil electromagnetic launcher capable of accelerating ~500 gram hollow projectiles to 150 m/s.