My guess would be a "resonance coil weapon" which I think was something http://en.wikipedia.org/wiki/Nikola_Tesla was rumoured to be working on.
Good call. I would think this the most plausible option given the heavy influence of Niko Tesla on FO lore. While looking up information about a resonance coil weapon I discovered this PDF which is really fascinating. It describes Rail Guns and the Gauss Rifle with information on how they would work. It is from a guy that has apparently built semi functional prototypes.
http://www.intalek.com/AV/Railguns-Tim-Ventura-1994-2.pdf