Pump-action shotguns are great in most situations. On human opponents, that clickety-clack working of the action gives you a huge psychological edge through sheer intimidation. They are easy to maintain and super reliable -- which makes them preferred weapons against slow moving zombies. And they are very versatile because they consume a variety of ammunition types.
But sometimes you need all the knockdown firepower of the shotgun with the rapid fire and easy reloading associated with an assault weapon. For those situations, you need the AA12 Automatic shotgun.