OK, I know it, even played with some of the camera mount M-16's in test during the 80's, although they didn't have the hinge, you could just see where you were aiming when you were not behind the weapon. I've also seen some of the old smooth barrel SMG from WWII designed for simular purpose that had a curved barel. Seen periscope rigs for Enfields to shoot from trenches so the gun was three feet above the shooter and you shot with a pair of mirrors.
Can't say say I see the big draw in them, I much perfer the long ballers and a handgun round is so marginal in performance it makes it little more than a fancy room clearer assuming you are standing behind a wall that is cover.
More intrested in the Kel-Tec bullpub riffle they are coming out with. now a 308 with over 30" of tube is exciting. Performance exceeds a 30-06 and is dam close to Mag standards. Hell even with the short tube it seriously changes what you can consider as safe cover in urban warefare.