If the 'C' stands for 'Charge' then it should be Electric Charge, as electronic refers to components, devices, systems, or equipment that use electricity.
It seems to me that it is unlikely to be Electronic Coil, as the ammo is clearly just supplying the energy that powers the device, plus the 2mm (likely tungston) projectile. The coil, or sequence of coils must be built into the body of the weapon and would be a major part of the structure.
Personally I would go with '2mm Electrically Charged', as it both makes sense semantically, and sounds OK.