The concept of putting a shotgun under an AR works FAR better in video games than it does in real life.
Every few years, a military unit decides that an underbarrel shotgun is the thing to have. KAC's MasterKey was originally designed to fulfull such a request, depsite KAC's own insistance that it wasn't a great idea. As has happened each time, once the shotgun is actually installed, and the "neat" wears off, the people who wanted it realize that they were far better off with a separate pistol-grip shotgun in the first place, and the MasterKeys get moved permanently to the arms room to gather dust.
If you want to know what a MasterKey is like, go find six 1-pound fishing weights and tape them to the bottom of your AR barrel, then try some standard mulitple-target carbine drills. If you still like the idea, you can still persue it. If not, you're out less than $10 instead of $2,000+.
-Troy