Headshots, Do they only work on zoomed in Sniper?

I was just wondering if headshots are even detected or is it just with the sniper zoom like TF2's sniper.

To me this seems very important for the Tank's Railgun...and perhaps Assault's Rifle.

They only count as an instakill on the sniper. But I believe they do extra damage on any weapon.

Head shots do cause extra damage. Just aim for the head of gunners or tanks with an assault class and you'll see for yourself.