Two years ago I was an intern at NASA working in a ballistic impact materials testing lab. We constantly had trouble getting an exact kinetic energy on our projectiles 2 kg @~ 500m/s = 250kJ. We were getting fluctuations of about 10kJ final kinetic energy resulting in many shots to get the exact energy levels needed. The propellant is pressurized helium with a maximum theoretical velocity of 972 m/s. I would like to build a multistage coil gun at the end of the light gas gun to act as a projectile speed controller, adjusting the projectile kinetic energy up or down to get closer to the desired value. Also, I would like to use the coils to achieve a faster maximum speed if desired. This could have some fun hobbyist aspects too such as an accelerator add-on to a BB gun. If anyone has any thoughts on this it would be appreciated.

Note: I am working to get an old capacitor bank from Los Alamos National Laboratory, specs are as follows:

Each Cap:
C = 55 microFarad
Vmax = 15kV
Westinghouse Type DV energy storage capacitor
Oil Filled

Current Bank Specs (adjustable):
360 Capacitors in Parallel
C = 19,800 microFarad
V = 15kV
E = 2.2 MJ