Always thought a shotgun would be awesome. Trouble is, look how huge shotgun shells are. You could carry waaaaaaay more normal cased bullets from 9mm to 5.56mm and a bullet is a bullet at the end of the day.
Not so, its fairly difficult to shoot a zombie with a shotgun without killing or at least disabling it, if your even semi competent its 1 shell = 1 kill, a 5mm round isn't even a guaranteed kill when straight to the face.
And lets not forget that we are in the UK, its not like the is a gun store on every corner with boxes of 9mm rounds, I would wager that shells are in more supply than bullets in this country.