Though I haven't tried them yet, a lot of people like 69 BTHP (OTM: Open Tip Match). I'm still loading 55 grs but will upgrade to 62 gr steel insert AP rounds when the 55's run out.
i have plenty of xm193', but just in case the zombies are lightly armored I have a supply of the m855 green tips...... I have a various assortment of other rounds too, mostly hunting rounds though.