Specifically: not a "space gun" for launching payloads to space. This subject has been discussed a lot, and I know of the slew of problems and their partial solutions enough.
I mean a device mounted on a craft, using electricity for launching macroscopic pieces of metallic reaction mass at hypervelocity, in the opposite direction the craft is meant to move.
Railguns operating in atmosphere have limited applications and their own slew of problems. Some of them should vanish in void, while others might remain or get exceberated. Nevertheless, if EDO-1 ever reached the planned 7000m/s exit velocity, it would constitute an engine of over 700s of ISp, and that's surely not the end of it - if we don't strive for heavy projectiles of aerodynamic shape and armor piercing properties, just focus on getting whatever shape ejected as fast as possible, this could see even further improvements.
Of course there would be problems. Pulsed propulsion stress, energy supply, heat dissipation and so on. But you have to admit 700s of ISp sounds very tempting.
And of course I'd be very surprised if I was the first to think of it. So - has this been attempted, developed, researched? Some unforseen problems that make it totally impossible or otherwise a misguided endeavor?