While short barreled assault rifles like the U.S. M-4 are favored for troops acting as bodyguards, riding in vehicles, or liable to do a lot of fighting indoors, there have long been complaints that the slanted pistol grip was not ideal for close quarters (like from the seat of a vehicle or inside buildings). There, a vertical grip was preferred. Now at least one firm has come out with a solution: the Tango Down Flip Grip. This looks like a standard slanted assault rifle pistol grip. But pull down the Flip Grip and rotate it 180 degrees and you have a vertical grip. Pull down and rotate again and your back to the traditional slanted grip.
Not a huge market for Flip Grip, but for those who do need it help is less than $80 away.