'Gauss gun' is a generic term for kinetic weapon that uses a magnetic charge to accelerate a projectile. Hand-held models tend to use a simple small calibre (usually 2 to 4 mm) high density solid ferrous slug which is accelerated to hypersonic velocity. Some variants use a combination of explosive and magnetic accelerant. These weapons rely on a very high rate of fire to make them effective. Large vehicle or ship-mounted gauss guns with open barrels are called railguns.