This source from 1995 says rail gun efficiency is 47%: https://ieeexplore.ieee.org/document/364679
Abstract: Railgun launchers must be energy efficient to realize use in practical systems, especially mobile applications which require compact power supplies. This paper shows that railgun launcher efficiency has already been demonstrated at 47 percent at launch kinetic energies up to 8 MJ. Improvements in armature performance have made this performance level possible. Foreseeable improvements in armatures and power supply-launcher integration will push launcher efficiencies to at least 66 percent.
