Shotguns are great guns. They allow for a lot of damage to be dealt in a single shot. They cripple, maim and kill, but they are not the best weapon for fighting Zombies. The reason is simple: the only way to kill a zombie is to destroy its head. Now, there is no doubt, that a shotgun will make a zombie’s brain pan explode like a melon, but there is a problem. Shots fired from them scatter, and they do not have much of a range. This means, that for the shotgun to be effective against zombies, the undead cannibals have to be close, and if that is the case you are already dead. And if they are not close enough for a guaranteed head shot, you are going to hurt your cause more than help it.
A zombie can still function with its limbs and torso destroyed. Its jaw will continue to work and it will never stop trying to close with you so that it can bite you. If there is only one zombie, this is not much of an issue as you can blow its legs and arms off and then safely approach to deal the killing blow to its head. The thing is, though, zombies are rarely found in isolation. They roam in packs, and the only thing you accomplish by blasting their legs then is to make it more difficult for you to escape. Sure, you can fill them full of shot, maybe even taking one or two down as you do so, but the beasts will still be moving, functional, and dangerous.
No, for killing zombies, single shot or semi-automatic weapons are the best, especially pistols since they do not require much room to wield, and can be used in one hand without compromising too much. See, when fighting zombies, quickness and mobility are essential. Shotguns, while cool, require two hands, and often need pumping. This means that the time between shots is too great. A pistol on the other hand, whether a revolver or a magazine equipped one had a rate of fire equal to how fast you can pull the trigger. Also, they generally hold more ammunition, are lighter, and easier to wield.
The point is that while shotguns are cool, they are not the most effective tool to take out a zombie, no matter what movies and video games tell us.