Damage Modifiers

Everything's 100% except the following:

Vulnerable to...

Incendiary:
    Reaper             170%

High-Explosive:
    Silacoid           130%

Laser:
    Sectopod           150%

Melee:
    Unarmoured Soldier 120%
    Civilian           120%
    Sectoid            120%
    Celatid            120%
    Floater            120%

Celatid Acid Spit:
    Unarmoured Soldier 160%
    Civilian           160%
    Sectoid            160%
    Celatid            160%     I find it amusing that Celatids
    Floater            160%     are vulnerable to their own spit -- Danial
    Personal Armour    110%

Resistant to...

Armour-Piercing:
    Cyberdisc       80%
    Muton           60%
    Zombie          60%

Incendiary:
    Chryssalid      80%
    Ethereal        70%
    Snakeman        70%
    Tanks           40%
    Personal Armour  0%
    Power Suit       0%
    Flying Suit      0%
    Silacoid         0%

High-Explosive:
    Sectopod        80%
    Zombie          80%
    Tanks           70%
    Cyberdisc       60%

Laser:
    Zombie          70%

Plasma:
    Sectopod        80%
    Zombie          70%

Stun:
    Personal Armour 90%
    Chryssalid      90%
    Power Suit      80%
    Flying Suit     80%
    Ethereal        80%
    Zombie           0%
    Tanks            0%

Melee:
    Tanks           90%

Celatid Acid Spit:
    Tanks           40%

Susceptible to...

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Ammo Type	Damage Modifier Category
Ammo Type	T+I	Hu+	PA	PS	Tnk	Snk	Eth	Mut	Sil	Chr	Rea	Sec	Cyb	Zom
Armour Piercing	100	100	100	100	100	100	100	60	100	100	100	100	80	60
Incendiary	100	100	0	0	40	70	70	100	0	80	170	100	100	100
High-Explosive	100	100	100	100	75	100	100	100	130	100	100	80	60	80
Laser	100	100	100	100	100	100	100	100	100	100	100	150	100	70
Plasma	100	100	100	100	100	100	100	100	100	100	100	80	100	70
Stun	100	100	90	80	100	100	80	100	100	90	100	100	100	0
Melee	100	120	100	100	90	100	100	100	100	100	100	100	100	100
Acid Spit	100	160	110	100	40	100	100	100	100	100	100	100	100	100

Column	Modifier Index	Unit Type(s)
T+I	00 / x00	Terrain and items.
Hu+	01 / x01	Unarmored Soldiers, Civilians, Sectoids, Celatids and Floaters.
PA	02 / x02	Personal Armor.
PS	03 / x03	Power and Flying Suits.
Tnk	04 / x04	Tanks and Hovertanks.
Snk	05 / x05	Snakemen.
Eth	06 / x06	Ethereals.
Mut	07 / x07	Mutons.
Sil	08 / x08	Silacoids.
Chr	09 / x09	Chryssalids.
Rea	10 / x0A	Reapers.
Sec	11 / x0B	Sectopods.
Cyb	12 / x0C	Cyberdiscs.
Zom	13 / x0D	Zombies.

To summarize re: the all important X-COM armor:

AP, HE, Laser, and Plasma: All types of X-COM armor have full/normal (100%) susceptibility to these types of damage
Stun: Personal Armor has 90% susceptibility to stun, while Power/Flying Suits have 80% to stun. (Less than 100% is better.)
Incendiary: Personal, Power, and Flying Armor are completely resistant to Incendiary.