They inflicted damage by hurling projectiles at high velocities, which imparted kinetic energy and force to the target upon impact. Damage was thus a result of the mass of the projectile and its velocity. For a rail gun (where the slug is accelerated by Lorentz forces and electromagnetism between two parallel rails) there was no real limit (except the speed of light), only a practical one based on rail erosion and, when in atmosphere, ablation of the projectile. Typically this speed was 6 km/s.