Been working on them myself
Lots of good tips people put in this thread
put laser on it and shoot from the hip aiming at the head, its a hip fire assist glitch.
Have you done the PDW yet? That one seems to take forever. I've been using it a lot for 90 levels and only got 60 headshots.
Hardly use the MP7, Vector or Peacekeeper and got 15 on the first two and 30 on the peacekeeper. I really have barely used them and they seem a piece of cake to get headshots with in comparison to the PdW.
Play hardcore and start at the chest. The gun tends to pull upward (at least for me). The Vector went pretty smooth for me for some reason. Only second to, believe it or not, the Chicom.
I know it sounds obvious, but aim for the head. Once I started going for headshots they came rolling in, usually got 10 per game playing HCKC.
update to this.
I switched to using primary gunfighter...........
using laser sight, long barrel, burst fire with the vector, and racking them up