I've always wondered who buys those (and the other "pistol" versions of battle rifles). A friend had one at the range once...it was a fun but useless ammo muncher. Adding a foregrip made it a bit more controllable, but it turns out that it is illegal as well.
Rifle ammo is made to go through a rifle barrel. Put it through a short pistol barrel and much of the energy of the powder is wasted as muzzle flash.