One of the advanced armor training options is steel headbutt.
Steel Headbutt (Ex)Source Armor Master's Handbook pg. 15
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While wearing medium or heavy armor, a fighter can deliver a headbutt with his helm as part of a full attack action. This headbutt is in addition to his normal attacks, and is made using the fighter’s base attack bonus – 5. A helmet headbutt deals 1d3 points of damage if the fighter is wearing medium armor, or 1d4 points of damage if he is wearing heavy armor (1d2 and 1d3, respectively, for Small creatures), plus an amount of damage equal to 1/2 the fighter’s Strength modifier. Treat this attack as a weapon attack made using the same special material (if any) as the armor. The armor’s enhancement bonus does not modify the headbutt attack, but the helm can be enchanted as a separate weapon.
This leaves some things open.
- what adds to the damage in addition to 1/2 strength?
- is the steel headbutt part of some fighter weapon group?
- is it condsidered a natural attack?
- can you do it while wearing a dwarven boulder helmet?
- if you use power attack, how much bonus damage does the headbutt do? +1 as an offhand attack/ a secondary natural attack; +2 like a onehanded weapon; or nothing?
What uncanny coincidence, I just finished building an NPC for tonight's game that uses Steel Headbutt. So the ability is fresh in my mind.
1) This would be on a case-by-case basis depending on the ability in question. I would imagine that most abilities that give a bonus to damage would apply, since there's no reason they wouldn't.
2) Arguably it could be considered an unarmed strike, in which case it would be in the Close, Monk, and Natural groups. Otherwise it wouldn't be in any weapon groups.
3) No, it wouldn't be. It could arguably be considered an Unarmed Strike.
4) By RAW you could make the Steel Headbutt but you wouldn't gain any benefits from the Boulder Helmet.
5) It's not specified, but given the way it's described it seems to be following the rules of secondary natural attacks so I would go with that.