So how much damage does each shot do?

To determine the damage of an weapon, apply the upgrade tree damage nodes as multiplicative.  Then add up your damage mods(don’t forget when you apply it to add 1 so the base damage stays).  For example, a level 30 Latron with 5 damage nodes and +22%/+21%/+20%/+17% damage mods.

48*1.8=86.4 normal damage(rounds to 86)

Now, if I were to add a 25% armor piercing mod(4 slots for damage and 1 for AP), I would do 86 normal damage and  21 armor piercing damage.
If I had a 6th mod slot(rifles only have 5 mod slots) and put in another 25% armor piercing mod, I would do 86 normal damage and 42 armor piercing.
If I had a 7th mod slot and put in a 25% freeze mod, I would do 86 normal damage, 42 armor piercing damage, and 21 freeze damage.

Shotguns fire a specific amount of pellets with a specific base damage.  HEK = [email protected] dmg, Strun = [email protected] dmg, Boar = [email protected] dmg.  Each pellet has it’s own damage calculation and crit check.  This means all upgrade nodes and mods must be applied to each pellet individually.  For example, a single +2% damage on a level 1 strun/boar does nothing(2% of 12 is .24, which gets rounded down to 12, or 0 in the case of elemental damage).

Elemental damage(AP/Fire/Freeze/Lightning) is determined by the mod value and your normal damage.  You do not get more damage by adding all the elements together(Using a weak element is around 1/4 to 1/8th the strength of an element strong against that opponent).  Fire should only be used against infested and lightning only against corpus.  Freeze has a nice secondary and decent damage. Armor piercing is great against anything except infested runners/leapers/chargers.

Crits take the damage of a hit and multiply it by crit damage.  Nothing is bypassed, no weird mechanisms.  All elemental damage is subject to the same multiplier(if a projectile crits, ALL damage from that projectile is multiplied by crit damage).  Shotguns calculate the crit chance on each projectile, as does multishot.  One thing to note, crit damage tree nodes are +10% multiplicative(2.2 crit damage multiplier with 4 nodes).  Crit damage MODS are additive.  +50% crit damage is only worth .75 multiplier, and +100% is only worth 1.5.  Sorry, you cannot get to 70x crit multiplier.

Shields and armor(i.e. stuff you have to deal with to kill mobs)

Shields-Take full damage from all damage except freeze, which does double damage.  There is no locational damage to shields(no headshots, sorry).  If you break a shield and still have damage remaining, 1 point of the remaining damage goes away and the rest is applied to health.

Armor(grineer normal/shield/elite, infested crawler body, Ancient body/head, Corpus crewman)-Reduces all damage by a base amount with a per level scaling reduction.

66% reduction for normal/shield grineer body shots
around 20% for grineer elite body shots(Rare spawns = few data points.  ;_;)
50% reduction for crawler body shots
33% reduction for nauseous crawler body shots
25% reduction for ancient head shots
75% reduction for ancient body shots
100% reduction for Corpus Crewmember head shots
20% reduction for Corpus Crewmember body shots

Armor ignoring melee weapons ignore all reductions for the normal damage they apply(flat or per level), Armor piercing elemental damage does flat damage subject to a multiplier, and piercing mods do not confer armor piercing.  If fire or lightning hits a non-vital area on an armored mob, it will be subject to per level decreases(but vital shots with a mobs elemental weakness are always flat)

Elemental(AP/Fire/Freeze/Lightning) is applied first.  If this kills the enemy, it does not count as being killed by that weapon.  If the target is not dead, normal damage is applied.  If it is a shotgun, each pellet is applied when it hits.  If at any time an enemy dies, damage will stop being calculated and displayed.  Therefore, if any elemental damage is enough to kill something alone, you will not see the remaining normal/elemental damage numbers.

Likewise, if a shotgun/multishot kills something and there are still pellets/bullets hitting, those bullets are now hitting a corpse and will not show damage numbers(They will however show blood spatters, sparks, etc)

Grineer-Everything armored except melee Grineer

Normals(trooper, lancer, etc)-Head weak point, armored everywhere(no flat reduction on headshots)
Headshots Normal= 100% – ~1.3%/lvl, AP=300%, Fire= 75% – 1%/lvl, Freeze= 33% – 1%/lvl, lightning= 40% – 1.25%/lvl
Bodyshot=  headshot normal dmg / 3 (armor), all elemental = half of headshot

Sawmen-100% on body shots, 300% normal damage and 200% elemental damage on headshots

Shield Grineer-As normals, but 2 levels behind

Bombadier/Gunner-Head weak point, armored everywhere(no flat reduction on headshots)
Note:Highest encountered was level 23
Headshots= normal = 200% @ level 8 – .5% level, AP = 200%, fire < 100%, lightning/freeze > 100%
Body= normal = headshot / 3, AP= 100%, fire/lightning/freeze= headshot / 2

Commander-normal= same as bombadier/gunner, immune to lightning/fire damage, AP/freeze not tested

Infested-Ancient heads/bodies and crawler bodies armored

Normals(chargers, leapers, runners)-Head weak point, no armor, staggered by fire
100% on body shots, 200% damage on headshots, AP & Lightning=50% body, 1x head, Fire= 2x body, 4x head,Freeze= 1xbody, 2x head

Ancients(Disruptor, Toxic, Healer)Lower legs/arms weakpoint, head lightly armored, body armored, staggered by fire, Immune to lightning
Feet/hands/shins/forearms= Normal 100%, AP=100%  Fire=200%, Freeze=100%, lightning = ZERO
Head=75% – 1%/lvl, AP=200% Fire=75% – .5%/lvl Freeze= 50% – 1.5%/lvl, lightning = ZERO
Body=Headshot normal damage / 3 (armor), AP/Fire/Freeze = half of headshot damage, lightning = ZERO

Nauseous Crawler-Head weak point(2x), forearms/hands/shins/feet weak point(1x), Bodies armored
head= 200% normal/freeze   AP/lightning=100% Fire = 400%
forearms/hands/shins/feet = 100% normal/freeze  AP/lightning=50%  Fire = 200%
body= 66% normal-.5%/lvl, AP = 50%, Fire = Slightly over 125%-?/lvl, Freeze= 50%-?/lvl, Lightning = 50%-?/lvl

Crawler-Head weak point(2x), forearms/hands/shins/feet weak point(1x), Bodies armored
head= 200% normal/freeze   AP/lightning=100% Fire = 400%
forearms/hands/shins/feet = 100% normal/freeze  AP/lightning=50%  Fire = 200%
body= 50% normal-.5%/lvl, AP = 50%, Fire = 100%-?/lvl, Freeze = 50%-?/lvl, Lightning = 50%-?lvl

Corpus-Crewmen armored

Crewman-armored EVERYWHERE.  Armor ignoring melee does full damage.  Staggered by lightning
Head immune to normal, AP=2000%, Fire=100%, Freeze=200%, Lightning = 400%
Body 81%-(.5%/lvl), AP=100%, Fire=33%-?%/lvl(damage too low to say) Freeze=75% – .33%/lvl  Lightning=160% – 1%/lvl

Moa – Weak point is midsection/gun.  Staggered by lightning
Shielded=100% normal/AP/Fire/freeze, Freeze=200%
Legs=100% normal/AP/freeze, Fire= 50%, Lightning=200%
Midsection/gun=200% normal/AP/freeze, Fire = 100%, Lightning =400%

Osprey 100% normal/AP/Fire/Freeze, 200% Lightning

Cameras-200% lightning.  Very very very low HP.

To wrap this all up:

Armor Piercing is very effective against the Grineer, always gives flat bonuses, and does well against pretty much everything(only less than 100% on non-vital hits on infested runners/chargers/leapers/crawlers).

Freeze doesn’t do good damage, but makes Ancients much easier to deal with(moving slower makes their feet/hands move around less, and their charge becomes hilariously slow).

Fire is a subpar damage source vs anything but infested(and only staggers infested)

Lightning is a subpar damage source vs anything but Corpus(and only staggers Corpus)

Normal damage is good when shooting at anything, as long as you know where to shoot.  Additionally, it is what is used to determine the strength of elemental damage.