I use a Hornady 5.56mm TAP round in my AR just in case of a zombie attack.
first off you should be ashamed of yourself for using 5.56 imo... you would greatly benefit from replacing that reciever to accomdate something more along the lines of a .308
... That being stated, though I completely trust the accuracy of the 5.56 fired from that platform at a distance, and it's effectiveness on a live able bodied target at that distance, It would not be my main zombat weapon
If you were curious, Mine is a little beauty named Helen, of the ak47 descent
and her bite is much worse than her bark
She rides duet with my 870 MCS chambered in 3" magnum, and with a little 1911 kimber action for the kicker
...
I can't wait... The question is not if it's going to happen, But rather when and are you prepared.