Another issue seemed to be that many attacks to the head damaged the muscle and fat, which around the skull are fairly insubstantial.
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Actually, DF heads are rather meaty in proportion to real-life heads.
There is actually a pretty quick way to change the attack power of most organic creature's melee attack.
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Put [ATTACK_VELOCITY_MODIFIER:x] (where x is less than 1000 for a weaker attack) in the creature's natural attack raws?
Okay so it's actually an issue with representation, since it doesn't actually indicate that the hits get through the "head muscle" wall.
Pulping notification is in the same part of the code as severing notification - it replaces the text that would have previously gone there (which would have mentioned the skull shattering).
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You will have a weirdly large volume of skin but none of that should be noticeable ingame.
Adding bone to the head will probably make announcements even weirder. 'you bash the human in the head with your mace, fracturing the bone and fracturing the skull!'.
Yes, part of the problem is that the size of the head controls volume (damage resistance), contact area, and thickness (penetration). The relative thicknesses of the tissue layers in the head then decide what all of that thickness is made of - the head doesn't get smaller if you remove the fat/muscle layers. If you make the head smaller to have less tissue, then the head actually also gets smaller and presumably harder to hit (and would look silly if you sketch the creature to scale).
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Also it would be a good idea to expand this mod to armor. From what I recall the stock armors have very irregular armor thickness and can be stacked in weird ways (4 mails coats and plate armor or something). It would give you more to balance the weapons against.
I think I documented it in a place or two, but several of the armor parameters multiply together, relative to the creature racial body part size, to determine the armor volume. The volume then leads to contact area, penetration thickness, and damage resistance.
I.e. stock gauntlets are much thinner than stock breastplates because the hands are smaller than the upper body.
Also, tissue thickness scales differently from clothing/armor thickness, leading to the absurdly thick clothing found on the giant-sized demon lords sometimes found running human civs. The (bugged?) clothing is thicker than the penetration value of any stock weapons.