Not a tester/shill/salesman for Adam (I just think this is a damn good design and can't wait to get one), but I can see a definitive advantage he neglected to mention in his post above. When loaded with shells these things are sized to fit into standard AR mag pouches. Presumably if you have an AR mag taco pouch, you can pull your elastic cord over to secure the Q-DC mag. Or if you pack a go-bag that holds AR mags, you can substitute Q-DC mags in their place.
This opens up, for those of us who prefer shotguns, a whole host of already capable gear. It allows considerable modularity. I have been holding back, for years, on using a shotgun as a regular go-to gun for travel, because there wasn't a good way to regularly carry 'enough' spare ammo. I have tried a lot of options. There are game/sporting clays type bags and vests where you just dump rounds loose into a bag, which doesn't keep them oriented or help you get the gun loaded. There are "speed loaders" that usually require modification to the gun to use. And there are bandoliers, elastic and nylon shotgun "cards" that hold from 6-12 rounds each those options are big, bulky, and floppy and don't really offer a lot of advantage over just a box of shells.
Of course the main source of spare ammo for a shotgun is on the gun, but the Q-DC offers a sturdy, compact, option that can utilize pre-existing gear more fully than old options, to refill that gun-mounted spare ammo. And that is brilliant, in my estimation.
-Rob