Headshot Multiplier: x5 (everywhere(dealt/received Singleplayer, dealt/received Multiplayer))
Damage received in Single Player: Arcade: x1 - Normal: x2 - Realistic: x3 - XXXX: x4
Damage dealt in Single Player: x4 (applies to all difficulties)
Enemy Body HP in Single Player: Arcade: x1 - Normal: x1.2 - Realistic: x1.8 - XXXX: x2.6
Enemy Head HP in Single Player: x1 (applies to all difficulties)
Damage dealt/received in Multiplayer: x2.4 (excluding Sniper which is x4.2)

Body Part Damage Multiplier: There are only 2 different body part multipliers in the game, one being the head and the other being everywhere else on the body, where i refer to as 'body', i am referring to anywhere that isnt the head.

Damage Drop Off: All weapons have damage drop off (excluding Sniper, Crossbow and Knifes) that drops off with a linear interpolation, however despite the damage being gradual, the amount of damage done will round up to the next nearest higher whole number(ie; if a weapon does 5.3 damage, it will actually do 6 damage instead), this is to ensure that weapons aren’t small damage amounts away from their breakpoints (ie: a 9mm will do 10 damage instead of 9.9 damage so that it isn’t missing its break point).

By using the above multipliers, you can work out what amount of damage your gun will do, but I have listed the most important damage amounts (XXXX difficulty and Multiplayer) for you. Number on left is damage that enemies deal to you in Single Player XXXX difficulty and also the amount that you deal to enemies on all Single Player difficulties (x4), number on right is damage dealt/received in Multiplayer (x2.4, excluding Sniper which is x4.2)

9mm: 10 – 6
Kalash: 10 - 6
Assault Rifle: 8.67 – 5.2
MiniGun: 8 – 4.8
M60: 18 - 10.8
Revolver: 32 – 19.2
Both Shotguns: 12 x 5 (60) - 7.2 x 5 (36)
Sniper: 35 – 41
Crossbow: 20

Enemy Body HP multiplier: Arcade: x1 - Normal: x1.2 - Realistic: x1.8 - XXXX: x2.6
Enemies Body HP oddly has varying amounts, however below is the amount of HP that most enemies have:
Enemy Body HP: Arcade: 50 - Normal: 60 - Realistic: 90 - XXXX: 130
Enemy Head HP: 100 (all difficulties)

Armor/Helmet:
(Where I mention ‘armor’, i am also talking about helmets as they both work the same.): Decreases health damage by 86.11r% (its such a odd amount, i don’t know why they didn’t just make it decrease your health by like 85% or something). Whatever damage that would be dealt to your non-headshot health without armor is doubled to armor and is then multiplied by 1.1, the end amount is rounded up to the nearest whole number (Maths: 9mm does 6 damage to health without armor. 9mm does 1 damage per bullet to health with armor, except for does 0 damage per 5 bullets to health with armor, 6 ÷ 5 = 1.2, 6 + 1.2= 7.2, 1/7.2 as a percentage is 13.89r%, 100% - 13.89r%, = 86.11r%.).

Example:
9mm vs non-armor: 6 damage to health - Someone with 100 health will now have 94 health.
9mm vs armor: 1 damage to health and 13 damage to armor - Someone with 100 health and 100 armor will now have 99 health and 87 armor. (Maths: 6 x 2 = 12, 12 x 1.1 = 13.2, 13.2 rounded to nearest whole number is 13).

If you shoot someone and their armor depletes to zero, the amount of health damage that they take is based on the % armor remaining, ie; if someone has 20 armor and your weapon does 40 armor damage to them, then the armor will only reduce the amount of damage that they take by 50% than the armor would normally reduce health by (43.05r% because armor reduces health by 83.11r%) because 0 is half way between -20 and 20

Headshots: Its unusual for games to have two different HP amounts for both head and body, and yet even more unusual for a damage multiplier to only apply to one of them parts, the head HP being the same across all difficulties but the body HP amount being more on higher difficulties means that headshotting is more important on high difficulty, because the body HP is higher on hard difficulty whereas the head HP isn’t higher on hard difficulty.

Sniper: The Snipers damage is inconsistent compared to other weapons because the damage multiplier for it in comparison between Single and Multiplayer doesn’t align with other weapons. As in all other weapons Multiplayer damage is x2.4 but the Sniper is at x4.2. This means that the sniper does an unusually high amount of damage in Multiplayer. Perhaps they made the Sniper do more damage compared to the other weapons in Multiplayer to make up for it not being as good in Multiplayer as Multiplayer areas are generally very CQC.

Health/Armor whole number discrepancy:
Weapons do precise amounts of damage to health, ie; they don’t do whole amounts of damage (ie; 7), they do amounts of damage after decimal point (ie; 4.5). The HUD doesn’t say the actual amount of HP that you have though, it just says the nearest whole number (ie; it will say that you have 28 HP when you actually have 27.8 HP).
Armor is oddly contrary to this though; Damage that should be done to armor actually rounds up to the nearest whole number. For example; 13.2 damage to armor will actually do 13 damage to it instead because 13 is the nearest whole number.