While in Houston, I spoke to good friend and combat veteran Jim Navarro. At dinner we discussed a concept he is working on. In short, its a short barrelled rifle (SBR), but w/o the paperwork normally needed.
Picture an AR-15 pistol. Most of us scoff at this as a wannabe's gun. But Jim has given this some serious thought. One can replace the little bity "pistol" buffer tube with a CAR-15 type buffer tube and still be legal. It also improves function dramatically...or so I'm told.
This buffer tube is long enough to provide a suitable cheek weld as well. To keep it on the shoulder, simply add a crutch tip obtainable in any medical supply store. This provides a stock "toe" to mount into the shoulder for firing.
Add a flat top with a forward mounted Aimpoint, or as Jim has added, a Docter Sight, and you have the makings of a very serios urban gunfighting tool. And what is even cooler is that it is perfectly legal anywhere an AR-15 pistol is legal and available.