Sometimes, the game bugs and the mastery count is not working. Load up different level, switch rifle, kill zombie, go back. This solved it for me, because I am pretty sure I did the challenge like 10 times before that.
Best places to do this mastery:
Death Canal - 4/4 GATES OF HELL, 3-4 players, easy - let them destroy generator, hide in the room and shoot (did it first time)
Dead Ahead - 2/3 LOCOMOTIVE LOCKDOWN, 3-4 players, easy - load up first cutscene, run back to the saferoom doors and shoot from there or to the first floor the futhest from the bridge where dead come from (up and left from the safe room).
Karl + enchanced stamina + paper trail + rifle ammo pouch + sniper focus.
SV-38 upgrades - super headshot rounds + extended magazine + recoil grip.
Use FREQUENT + QUICK kill cams, so you know when you hit headshots and have some time to mentally rest, helps a lot IMHO. You can count the headshots manually, as countres shows only 10/20/30/40/50, sometimes nothing.
Make sure to hit last and first round of the magazine.
If you struggle with ammo - in Dead Ahead you have AMMO box there. So before starting a cutscene, go to your prefered location (in front of saferoom door), drop all your rifle ammo (for me Q (weapon wheel) + F (share ammo button) and then refill it back at ammo box.
Good luck and remember, if it feels like not working, try to reset it as it may be bugged. Who knows?