A coilgun is a type of cannon that uses a series of electromagnets to accelerate a magnetic shell to very high velocities. Coilguns are often mistakenly called railguns by many sources, and while they are similar in general concept (that is, a magnetic gun), they differ in operation. Coilguns are essentially identical to mass drivers, though on a smaller scale.
Many hobbyists use low-cost rudimentary designs to experiment with coilguns. One such design would incorporate the use of photoflash capacitors from a disposable camera as the energy source, and a low inductance coil to propel the projectile forward.
In common coilgun designs, the "barrel" of the gun is made up of a track that the shell rides on, with the driver magnets around the track. Power is supplied to the magnets from some sort of "fast dump" storage device, typically a battery of high-capacity capacitors.
The power must be delivered to each successive electromagnet with precise timing, due to hysteresis. Magnets take some time to "charge up" after power is delivered to them, so the power supply must start before the shell has reached a particular magnet. The same is true after the power is turned off, and if the shell is on the "far side" of the magnet at that time, the magnet will continue pulling on it, slowing it down.
One obvious solution would be to trigger the magnets long before the shell reaches them, but because magnetic force drops off with the cube of distance (that is, very quickly) too much power would be lost with such a solution. For this reason most coilguns that use more than one magnet include some sort of electronic timing device for powering the magnets, one that can be adjusted for various parameters such as power of the shot, and the weight of the shell. The gun starts with all of the magnets turned on, and then turns them off one by one before the shell reaches them.
A superconducting version of the coilgun is called the quench gun. Resistors attached to superconductive coils waste energy in the coil, which is turned into heat. After a time this heats the superconductor up to the point where it is no longer a superconductor, thereby changes its state to normal (non-superconducting). When this happens the resistance of the coil as a whole suddenly increases, dumping all of the power as heat at a very rapid rate. By carefully controlling the heating rates, the magnets can be "turned off" in sequence at the proper rates to make a coilgun, one that generates very powerful magnetic fields with high efficiency, and tends to have lower hysteresis due to the rapid "burn out" of the energy in the coil.
One advantage of the coilgun over the railgun is that it can be made arbitrarily long. This has a number of side effects, but the main one is that the acceleration can be much slower over a longer length, meaning that the power needed in any one section of a coilgun is much lower. However this advantage is offset by the cost and complexity of the switching system needed to supply a longer gun.
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