M4 Carbine. At least you will have the zombie apocalypse covered and you could take it hunting if you really wanted to.
The M4 requires a bit more of a close up doesn't it? 5.56 mm?
I got to tell you, and we use them in the military, I would rather have something with a bit more power to it. In our version we're shooting an amped up .22 IMO. Not being an expert myself.