From TF2 Wiki
A Headshot is a successful shot to the head with a scoped Sniper rifle, Huntsman arrow or Ambassador bullet. Snipers who land a Headshot with the Sniper rifle score a Critical hit, though twenty-three milliseconds must pass from the time you use +attack2 (Scoping in) to the time you fire for a headshot to be considered a Critical. Players earn 1.5 points for getting a Headshot kill: one point for the kill plus half a point for the Headshot itself. Scoring a headshot with the Huntsman yields another half point in addition to the aforementioned bonus, resulting in 2 points per kill. A fully charged Sniper rifle headshot results in an instant kill (even on a overhealed Heavy). A headshot with the ambassador results in a Critical hit. Killing an enemy with an ambassador headshot yields an extra half point in addition to the kill.
The Sniper rifle, Huntsman and Ambassador are the only weapons to hit the head for bonus damage, though Pyros with exceptional aim can score headshots by reflecting arrows with the compression blast.
- A headshot with a reflected Huntsman arrow will show the Sniper rifle headshot icon, as there is no custom icon for this type of death as of yet.
- The 23ms delay for headshots was added to prevent from players from being able to predictably quickscope, a technique often used in other FPSes like Counter Strike.
- Killing a player with a headshot will sometimes cause a special death animation appropriate to that class (for instance, the Pyro does a backflip, the scout falls down stiff, etc.). This animation has a 25% chance of occurring.
- Headshots cannot do any crits to a Soldier or his party who is under the effects the Battalion's Backup.