Swen was talking about this one . . .
in a post here, describing it as the Colt IMP.
Bushmaster commercialized it briefly as the ARM Pistol. It is a bullpup design that rotates the whole weapon, minus the grip, about the barrel, so when fired right-handed it ejects to the right; fired left-handed, it ejects to the left, either way, it threw empties away from the shooter's body. It fired the full five-five-six from AR15 magazines. As the photo suggests, it borrowed many other parts as well from the AR15. Not so long ago, Bushmaster's catalog listed some few parts left for it, their website today does not.

When I was ready to get one in the pre-'94 AW Ban days, I had the three-hundred or so dollars that Shotgun News said the distributors wanted, and a dealer ready to order it for me with his nominal cut, but he could find none. I got a 9mm MAC-11 instead, and promptly regretted it.

Because the grip, with its trigger, moves with respect to the rest of the weapon and is also dislocated, so to speak, well forward of where it usually is put, it is difficult to make the trigger crisp. Maybe a solenoid affair can be made. Or hydraulics?