A zombie can only be killed with a headshot. Fully automatic weapons are are both impractical and a waste of ammo when that's considered.
Also, if there were such an event, martial law would be very likely (see any zombie movie for example). If you were walking down the street with either of those you'd draw a lot of attention from the military types...the wrong kind of attention.
Head shots are not a problem for me (legend in my own mind and maybe some extensive training as well) and I would certainly use the selective fire/semi auto. I agree full auto is a waist.
As far as the military/police who's to say they aren't zombies as well.
But to return to topic, don't you think that it is one hell of a rifle?