I followed the guides here and the last gun was supposed to be the Kalash 2012. It didn't unlock the achievement when I got my first kill with it. It meant that I was missing a kill with a weapon, even though I had been checking them off as I got kills.
I continued playing till Sparta changing guns as I saw them but what unlocked it was a knife kill, which was one of the first kills I got. In fact I think my knife kills were in the room where you first meet Hunter.
Im not sure why the knife kills didn't count when I originally did them. Im wondering if the kills you get during that first encounter with Hunter do not count for some reason.