Underslung Shotgun For The Ar?

Hey all. For a variety of reasons I'm thinking of adding an underslung shotgun to one of the ARs. It will require a tax stamp of course, but whatever. I've found the Masterkey to be, frankly, too unwieldy, so I'm thinking a single shot affair may be better than an abbreviated pump.

Alternatively, a duly registered M203A1 (40mm grenade launcher) could be adapted to 12-gauge and 00-buckshot rounds are available in the original chambering. They are hard to come by though; both the launcher and the cartridges.

Thoughts on options?

 

My fist thought would be the added weight. If your stepping down to a single round of 12ga, is it worth the added weight? The 203 was one round although it was used for a very different purpose. I like 12ga round for close quarters, but if in that situation the single round might be the not enough to justify the total added weight of the weapon. I like the idea, just not sure. Convince me