HSR is Head Shot Ratio. While you'd think it's how many headshots you get in total, it's a bit misleading. It's how many people you KILL with a headshot. So if you hit a guy with 4 headshots and shoot him once in the leg (and the leg shot kills him) that equals to 0% HSR. if you do the opposite (4 shots in the leg and one in the head (headshot kills him)) that's 100% headshot accuracy.