An object is pushed through successive magnetic fields, picking up velocity with each one. I have to think it would be a small fraction of the kinetic energy imparted by the rails. As Dennis already said. Re rotary intertia and power - interesting. As it seems right now, railguns seem to be the viable option for weapons applications, considering the navy is to my knowledge not testing full-scale coil guns. Coilguns and railguns are only really alike in that they use electromagnetism to accelerate their projectiles, rather than explosives as in a conventional firearm. I had not considered where the kinetic energy stored as rotation goes when the projectile hits. A coil gun is an electromagnetic accelerator. The projectile MUST be a magnet, or at least be magnetic. Yes, on quantum levels. Efficient would mean coilguns would not be used for large objects or high speeds. For things like point defense you would use a coil gun, for a relativistic projectile, a single shot rail gun, which would be designed to vaporize the rail as the projectile passed, could be more useful. A coil gun is a application of a linear magnetic motor. Coilgun vs railgun are so different it is hard to address both with same concepts. Alternating coils pull the projectile through the barrel with magnetic force.