I think this whole thread was a troll.
There is no reason not to use the infamous 12-gauge pump-action shotgun that we've all come to love through our conquests in L4D.
It's a perfectly suitable weapon at close-quarter combat against zombie hordes.
It has the power of a fully-swung baseball bat, and can knock anyone's head off with a single blast.
So I ask you, what is the point of this discussion?