I've wondered this, as well. Double barrels actually start to make a lot of sense when you go down below 14 inches. Since they don't have an action, you can get the overall length very short, with relatively long barrels. And when you're talking about a gun that short, competing pump actions only hold one or two more rounds and are heavier and not too wieldy or easy to pump.
Now I wouldn't do it to a "firearm," though. It would kick like a mule and you couldn't aim it. Basically be a breaching tool and nothing else at that point.
But I would absolutely SBS a double barrel and cut those barrels to around 8-10 inches, and take a good bit off the LOP, as well. In fact, if I were to ever SBS something, that's what I would do. Fact is, a shotgun can't hold a candle to a Mk18 for raw killing power in close quarters. But if I could get something significantly shorter and lighter than a Mk18 that had significantly more power than a pistol-well who wouldn't want that?
Here's a prime example:
https://cdngunblog.files.wordpress.c...ider.jpg?w=362
The overall length is about the same as something like a Serbu, but the barrels are several inches longer, and it only sacrifices one round in terms of capacity. You also have to take into account that you don't have to pump that round into the chamber, which is a very tangible benefit considering the poor ergonomics of pump actions that short, and the poor reliability of semi autos with barrels that short.
You can also well imagine how short it would be if the barrels were cut down to the 6 inches of the Serbu style shotguns.