Dunno about mounting under an AR, but I'ld bet it could mount up under a levergun pretty smoothly without the whole thing being terribly long (cause you don't have to put it ahead of a magazine well, you only have to clear the lever). I'ld lose the pistol grip and kinda just blend it back to the rifle's profile.
Practical applications? Naw, but mounted to another gun, you only have to worry about barrel length as overall length should be enough and you can shoot pretty much anything you want out of it without it automatically becoming a destructive device (unlike flareguns). I'm sure it could do wonders in a long hallway, but i don't want to be in that situation.