At close-quarters you don't even need to aim: pump-actions turn you into a chick-chacking death machine, while double-barrels convert a crowded hallway into an escape path lightly misted with Eau De Corpse. Undead ups the ante with a triple-barreled monstrosity which can only have been built by MacGuyver's evil twin.
That's 300 percent more phallic than an actual penis.
You're now a zombie because:
A shotgun has a worse "safe area" to "huge noise area" ratio than a Space Shuttle launch, which at least takes people places zombies can't follow. Firing a shotgun clears a few meters ahead of you while summoning everything in a mile radius to fill it again, so every shot generates a net increase of zombies.