coilgun

This is what the guys from the military-industrial complex devised when they realised that the railgun just doesn't work too well. Basically a great many coils aligned in a tube-like manner, with insanely large capacitors to power them. There are also ingenious rotary generators that can be used instead of house-sized capacitors. A computer energises the coilgun's segments in a carefully programmed sequence to launch a metallic projectile out of the thing's muzzle.

Advantages: There's no wear on the barrel, and the process is fully under control from the moment the trigger is pressed up to the moment where the projectile leaves the muzzle.

Disadvantages: No impressive side-effects.

In theory, coilguns could be used to propel stuff into orbit. In the 1940s, a crackpot German engineer proposed that the German artillery (which has always liked megalomanic ideas) should build a rigidly-installed coilgun somewhere to put England under a continuous barrage of cheap, but mean high-explosive shells. The immense amounts of power needed would have required that this thing be installed next to a coal power plant, which, in turn, would have required to be installed next to a coal mine in France or Belgium or such. However, the plan was condemned to fail because the computers necessary to control a coilgun didn't exist at the time.

The coilgun is one of the two most common electromagnetic projectile accelerator designs, along with the railgun.

The coilgun consists of a number of electromagnetic coils arranged in a line down the barrel of the weapon. By sending a current through a coil, a magnetic field is formed, and this is used to draw the magnetic projectile towards the coil. Once the projectile is at or near the center of the coil, the current is turned off, and the next coil has its current turned on.

Coilguns have several advantages over railguns, as well as a few disadvantages. First of all, it is a contact-less design. There is no need for the projectile to be in contact with any part of the coilgun while it is being fired, which means that energy loss due to friction is practically non-existant (assuming vacuum in the barrel). On railguns, friction may cause severe problems, and the stress of the acceleration may damage the rails.

Secondly, it can be built to an arbitrarily large size. Among other things, this means that one could build a very large space-based coilgun and use as a mass accelerator even for comparatively fragile implements. The great size of the accelerator would mean that one could reach a high speed without having to resort to a high acceleration.

On the negative side, a coilgun requires fairly sophisticated controls over when the current in a coil is turned on and off, as well as, if we're talking about larger designs, some positively huge capacitors.

There are several other concerns with such implements as well, but as I am far from an expert in the field I will not go in to these and risk huge inaccuracies.