A railgun is a device that uses electromagnetic force to launch high velocity projectiles, by means of a sliding armature that is accelerated along a pair of conductive rails. It is typically constructed as a weapon and the projectile normally does not contain explosives, relying on the projectile's high speed to inflict damage. The railgun uses a pair of parallel conductors, or rails, along which a sliding armature is accelerated by the electromagnetic effects of a current that flows down one rail, into the armature and then back along the other rail. It is based on principles similar to those of the homopolar motor.
Railguns are being researched as weapons that would use neither explosives nor propellant, but rather rely on electromagnetic forces to impart a very high kinetic energy to a projectile (e.g. APFSDS). While explosive-powered military guns cannot readily achieve a muzzle velocity of more than about 2 km/s, railguns can readily exceed 3 km/s, and perhaps exceed conventionally delivered munitions in range and destructive force. The absence of explosive propellants or warheads to store and handle, as well as the low cost of projectiles compared to conventional weaponry come as additional advantages.
Notwithstanding the above advantages, railguns are still very much at the research stage and it remains to be seen whether or not railguns will ever be deployed as practical military weapons. Any trade-off analysis between electromagnetic (EM) propulsion systems and chemical propellants for weapons applications must also factor in the novelty and complexity of the pulsed power supplies that are needed in electromagnetic launcher systems.
In addition to military applications, NASA has proposed to use a railgun to launch "wedge-shaped aircraft with scramjets" to high-altitude at Mach 10, where they will then fire a small payload into orbit using conventional rocket propulsion. Alternatively, the extreme g-forces involved with direct railgun ground-launch to space may necessarily restrict the usage to only the sturdiest of payloads or very long rail systems to further reduce launch acceleration.