I have at least killed more than 250 grunts via headshot, and I have been playing on Legendary and I haven’t seen the nameplate drop. Any ideas?
You need to wait until at least the next day. They don’t unlock once you hit 250. It unlocks on the next day which is usually 11a eastern time.
As above OP, this is a manual grant by 343. You’ll get it soon 
Cloing this topic off as we already have a number of topics discussing this right now