Wikipedia: A coilgun (also known as Gauss gun, Gauss cannon or Gauss rifle) is a type of magnetic accelerator gun (MAG) or magnetic accelerator cannon (MAC). It uses a series of electromagnetic coils to accelerate a magnetic shell to very high velocities. The name "Gauss gun" comes from Carl Friedrich Gauss, who formulated mathematical descriptions of the electromagnetic effect used by coilguns.
The gauss gun in this video shows how it requires manual placement of the projectile.
The devices here also require reloading, here the ball smashes directly into the magnet. It does illustrate the longitudinal principal. When an object is attracted to a magnet the force of the attraction can be extended to do work outside the magnetic field by attaching a mechanical medium to the magnet The medium will clearly display the work done by the magnet. The common argument here is that moving away the object attracted to the magnet requires just as much work at the attraction unleashed. As I've shown with the chainreactor this can-not be applied as it's using the object it-self as the mechanical medium the lever and as the projectile.
This is one with electromagnets.
This more-dimentional desing uses magnets in different sizes.