What is the difference between a Mass Driver & Rail Gun?

Very often it seems that people confuse a Mass Driver, and a Rail Gun. While they do have a similar purpose, accelerating objects to high rates of speed quickly…. There are two different methods to do this.

Let’s try to end some of the confusion.

Mass Driver / Electromagnetic Catapult / Coil Gun / Gauss Gun

A mass driver or an electromagnetic catapult use linear motors to accelerate and catapult objects to high speeds. A mass driver may also be referred to as a Coil Gun or Gauss gun, but those devices are designed with a ferromagnetic (magnetically reactive) payload in mind.  In other words, what ever is being accelerated has to be affected by a magnetic field.  The payload does not have to generate a magnetic field. If a mass drive needs to accelerate an non-ferromagnetic mass, than a carrier mechanism is typically used.  This carrier could be something like a sabbot casing/round, or an open ended “cage”, but if a carrier is used, then typically it is reused.

Does that clear it up?  Well, you might be asking what a linear motor is.  So let’s step back a step.

A Linear motor, effectively is a motor that has been “unrolled” so that instead of producing rotational force (Torque), it produces forward motion (linear force).


The illustration above, demonstrates a coil gun, but the concept is the same.  Each bank of magnets are phased to accelerate the payload.

Rail Gun

So, what is different with a rail gun?

A rail gun is a gun that works completely on an electrical basis.  There are two metal rails that are electrically charged, and the “shell” or payload bridges these rails.  So the payload / shell completes the circuit, that current interacts with the magnetic fields generated by the rails, and that accelerates the projectile.


The completed circuit resembles a large inductor, in that it is a large conductive loop with current through it, whose inductance is proportional to the area enclosed by the loop. The magnetic field generates a force upon all the components of the rail gun, but since the projectile is the only part not rigidly fixed, it is moved by the force. The force acts to increase the size of the inductive loop, driving the projectile away.

The key component to note here is that the projectile needs to be conductive, not ferromagnetic, and the rails must be exposed in order to pass current. This limits military applications because the presence of dirt in the rails could break the circuit, causing an electric arc flash, causing the system to act more like an arc welder. Also, the rails wear out due to the heating caused by the lack of superconductivity.

So, a true rail gun may not be practical for Military purposes, but a Mass driver / Coil Gun design would be more practical for Military purposes.