[EDIT] Uhg. Never mind. Just a few kills later, and I get my 50. I guess I was just extra cautious because I wasn't getting those milestones.
I'm currently on my second playthrough, and I've been sticking solely with the 92FS as my sidearm, going out of my way to get kills using the gun.
As we know, as you hit "milestones" towards the goal, it'll pop up saying how many kills you've gotten so far; even after my 250 headshots, it still rings up, showing if I've hit a milestone.
I'm on chapter 17 right now, and I have not seen any milestone notification, and I don't have the trophy for the 92FS (I'm sure I've killed over 50 people over the course of 1.5 playthroughs).
Has anyone had any issues with the weapon trophies? Is there a stats screen that shows how many kills I've gotten with each gun?