You really should specify a game.
Going from your posting history, this question is about TF2. As far as I can tell, TF2 only supports hitgroups (and thus headshots) on 3 weapon types:
- Sniper Rifles
- Bows
- Revolvers
Of those, the first two support headshots without changes, although you can disable them using the "sniper no headshots" attribute (attribute 42) on the weapon. Revolvers need an attribute to do headshots: "revolver use hit locations" (attribute 51).
Both of those attributes do the same thing; they change the firing mode of the weapon to its second (1) mode so you can't just slap them on other weapons and expect them to work.
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