What's better than an AR15? How about a semiautomatic shotgun? What's better than that? How about combining those things into one shooty death machine?
The idea of slinging a shotgun underneath a full-size assault rifle has been around since at least 1986. It was developed and used by actual elite Army units, like the 75th Ranger Regiment, who referred to it as the "Master Key."
It got its name because the intention was to use the shotgun as a breaching tool to blow doors off their hinges. Latching the shotgun on the rifle kept the tool out of the way and allowed a quick transition to the rifle for the breacher.
While breaching rounds aren't designed for antipersonnel use, I'm sure they hurt quite a bit. Not to mention that one of the great strengths of the shotgun is the ability to shoot a wide variety of nasty ammunition. Nothing is stopping a Master Key shotgun from feeding solid slugs or buckshot, or beehive shells. All of which can be quite useful when dealing with crowds of the undead in confined spaces.