This one is real easy, it doesn't say it but you can use any weapon for the headshots rifle, pistols whatever but obviously the rifles are the easiest to get headshots. I suggest the M1 Grand as there is 8 shots in a clip and had a high rate of fire with a fairly quick reload. Also to aid to the headshot process hold
and it'll slow down the zombies into focus time with will help a lot with this.
Good Luck!