 Even at 50% efficiency a rail gun launching a projectile with a kinetic energy of 20 MJ would require an energy input into the rails of 40 MJ, and 50% efficiency has not yet been achieved.To put this into perspective, a rail gun launching at 9 MJ of energy would need roughly 32 MJ worth of energy from capacitors.Current advances in energy storage allow for energy densities as high as 2.5 MJ/dm³, which means that a battery delivering 32 MJ of energy would require a volume of 12.8 dm³ per shot; this is not a viable volume for use in a modern main battle tank, especially one designed to be lighter than existing models.