There's the achievement that you need 50 headshots for lvl 1, but it's not working right.
I've got over 300headshots and I've only used the Scar, but it's showing only 52 headshots there. What's happening?

bump

So you did get the achievement?
Sorry, haven't done that achievement. I think it's safe to assume something about the registration of headshots (maybe hit detection in general) is off... can't tell you what, I haven't noticed it. If I have time I'll see if I can detect such an issue.