Multi-shot weapons fire multiple separately aimable shots.
Damage per shot equals (total multi-shot attack damage) DIV (number of shots in the multishot attacks).
If the total damage of the multi-shot is not divisible by the number of shots, the firsts shots will get +1 damage. Specifically the first (total multi-shot attack damage) MOD (number of shots in the multishot attacks) shots will extra +1 damage.

Examples for multi-shot weapon that fires 3 shots:

Total attack damage 10 will be split 4 3 3 per shot.
13 = 5 4 4
14 = 5 5 4
16 = 6 5 5
The ability to split the attack damage into multiple aimable shots can be sometimes usefull (blowing explosives takes tiny amounts of damage, detonate an oil patch with the first shot and use the fallowing ones to finish off multiple injured enemies per turn, ..)

Double shot(Seabrass & Payroll) and loose cannon(Beatrix) doubles the weapon's base damage ( BUT NOT ANY OF THE SITUATIONAL DAMAGE BONUSES) and double all "shots" (will fire twice for every aimable shot of a multi-shot weapon). As hinted in their cards THOSE SKILLS HAVE SIGNIFICANT BUILT IN AIM DEVIATION between the doubled shots. With the accurate handguns double headshots are impossible even at point blank range. This makes it a close range ability (AIM centre mass) or medium range vs large targets (boss or miniboss).

Damage per shot is split in 2 separate sub-shots. (total shot damage)/2. If (total shot damage) MOD 2 = 1 the first sub-shot gets the extra +1 damage.

Can be used to do extra damage vs boss/miniboss or to hit multiple targets with multi-projectile weapons since it doubles the number of projectiles. (destroy weak cover with some projectiles and hurt now exposed target behind it, kill weak target with few projectiles and hurt now exposed target behind him with the rest ..)

Some weapons split a shot into multiple projectiles with different firing patterns (ex bullet spray, scatter fire, burst fire) which makes them most effective at short range. Of those Burst fire is the most accurate at medium range.
Damage of the shot per projectile is again split in a simliar matter = (total damage of the shot) DIV (number of projectiles per shot). The first (total shot damage) MOD (number projectiles per shot) shots will extra +1 damage.

Examples :
6 projectile single shot shotgun - Total shot damage 14 will be split 3 3 2 2 2 2 per projectile.
4 projectile single shot SMG - Total shot damage 14 will be split 4 4 3 3 per projectile.

The ability to split a shot's damage into multiple projectiles can be useful when shooting trough weak cover or multiple weakened enemies. Ex few shotgun/minigun/etc projectiles are bounced but take down the cover so the rest of the shotgun/minigun/etc projectiles go through to hit the now exposed target. Especially effective with double shot/loose cannon that double the number of projectiles. Same principle can be sometimes used against lined up enemies - kill first with a few projectiles - damage the one behind him with the rest.

Sally's ability KILL SHOT/MAD DOG Will ONLY USE THE FIRST SHOT of multi-shot weapons (SADLY) which is way less than 100% weapon damage of multi-shot weapons). In ORDER TO RELIABLY CHAIN KILL SHOTS AT RANGE YOU WILL NEED TO USE A NON MULTI-SHOT, SINGLE PROJECTILE HANDGUN ON SALLY. (single projectile handguns are way more accurate at range compared to assault class weapons). To understand the power of this ablity: all situational damage bonuses (ammo, war cry, berserk, inspire) remain in play for each granted kill shot and with mad dog there can be kill shots as long as there are enemies left standing. (> lvl 2) With proper weapon, damage boosts AND clear line of fire to the targets Sally CAN CLEAR ROOM FULL OF MOOKS ALL BY HERSELF.

For Billy's ability Filet-o-furry (two consecutive melee attacks vs same target) also all situational damage bonuses (glove, war cry, inspire) remain in play for each of the two attacks, but since the ability is single target and in melee range - the potential for (ab)use is much more limited.

Most weapons on this game have fixed base damage. Very few starting game weapons and the arced shot explosives have RNG base damage. Don't know the mechanics that goven this (is it 50% chance to get the +1 to base weapon damage ?). Probably this optional RNG roll happens once per attack before all the other damage calculations are made. (so this +1 to base weapon damage, when it happens should ALWAYS get doubled by double shot/loose cannon )

Situational damage bonuses. This bonus damage IS ADDED ONLY ONCE PER MULTI-SHOT, DOUBLED SHOT MULTIPLE PROJECTILE ATTACK AND THUS SUBJECT to the same damage spliting per multi-shot/double-shot/multi-projectile mechanics.

INSPIRE (Piper aura):
Piper +1 lvl 2, +2 lvl 9.

POWER SHOT(Piper):
Piper +1 lvl 0, +2 lvl 7, +3 lvl 10.

WAR CRY(Sally AOE)
Sally +2 lvl 7.

BERSERK(Sally)
Sally +2 lvl 5, +3 lvl 10.

FOCUS(Valentine & Beatrix):
Valentine +1 lvl 1 , +2 lvl 7, +3 lvl 10.
Beatrix +1 lvl 1, +2 lvl 10

PAYBACK(Seabrass & Ivanski)
Seabrass +2 lvl 1, +3 lvl 7, +4 lvl 10.
Ivanski +2 lvl 1.

FLANKING(Payroll & Dora)
Payroll +2 lvl 1, +3 lvl 7, +4 lvl 10.
Dora +2 lvl 1, +3 lvl 10.

DAMAGE AMMO(item)
+1 +2 +3 to damage (often necesary for 1 mook kill shot capability).
+2, +4 to crit damage (situationally useful on lvl +7 Dora for scientfic precision).

DOUBLE SHOT/LOOSE CANNON (Seabass lvl 5, Payroll lvl 5, Beatrix lvl 5)
*2 base weapon damage. Significant amounts of damage with early game weapons that scales with weapon tiers: for example endgame shotgun has 13 base damage; endgame minigun has 14 base damage.

HEADSHOT(typically non-shotgun weapons)
As listed in loading screen hints headshots provide 50% chance to add the headshot bonus damage bonus to the TOTAL DAMAGE OF THE ATTACK. Some weapons, typically shotguns don't have any headshot bonus.
Chance to score a crit is rolled per head hitting projectile. (you need to hit the head/weakspot area).
All long range sharpshooter weapons get 75% chance (listed on weapon card). Glasses up non long range sharpshoter weapons to 75% and the long range sharpshooters to 87% .
Dora's level 7 ability scientific precision allows her to trigger the headshot bonus when hitting targets in the back (pretty easy to a lot of times since she can use sharpshooter weapons).

To repeat myself, the headshot damage bonus like all the other situational damage bonuses IS ADDED ONLY ONCE PER MULTI-SHOT, DOUBLED SHOT MULTIPLE PROJECTILE ATTACK AND THUS SUBJECT to the same damage spliting per multi-shot/double-shot/multi-projectile mechanics.

For example a SMG with +2 to crit damage firing 4 projectiles per shot.
If any of the first 2 projectiles crit it will do +1 damage.
If any of the last 2 projectiles crit they will do +0 damage anyway. (in this case the weapon's crit damage was not big enough to grant +1 damage to all projectiles)

Cover Mechanics:
Adjacent cover made transparent when you shoot trough will remain transparent till the end of the shot. You can't blow your explosive cover with riccoshet from weapons that don't do friendly fire.

Full health cover obstacles have hidden(sadly) amounts of hitpoints. Once damaged once they will reveal the remaining amount of hitpoints.
Most weapons projectiles bounce of cover similarly to how they bounce from solid-indestructable portions of the enviroment. After bouncing from cover the projectile's damage is substracted from the remaining cover's HP. Explosive cover has 2 hitpoints (can be blown up even with side-arms shots) and will kill most mook enemies in 2 tiles radius of the explosion when exploded by the player but will do reduced damage to the player's party.
Non-explosive cover have varying amounts(by model) of hit points, some of them can be very sturdy and is often a waste of time trying to shoot trough it with bouncing projectiles. However, vs significantly weak(weakened) cover shooting multiprojectile weapons at the target might make sense (Ex few shotgun/minigun/etc projectiles are bounced but take down the cover so the rest of the shotgun/minigun/etc projectiles go through to hit the now exposed target. Especially effective with double shot/loose cannon that double the number of projectiles. Same principle can be sometimes used against lined up enemies - kill first with a few projectiles - damage the one behind him with the rest .)

Very few, very rare weapons and Fen's Lightning Coil trait have the ability to pierce cover. Damage to cover health is dealt as per usual - but instead of bouncing the projectile/s go straight trough cover.

Complex numeric example.

lvl 10 Payroll dual shot shooting a multi-shot multi-projectile weapon The Baroness rare SMG with +2 damage radioactive ammo.

Base Damage 6, +4 crit, 2 "attacks" 3 projectiles per shot.

Weapon Base weapon damage 6.
Situational damage bonuses +4 flank, +4 crit.


Damage numbers for non-crit, non-flank
Total Attack damage = 2*6 (doubled base weapon damage since we're using dual shot) + 2 (ammo.) = 14

Multishot stage. We have 2 shots per multishot. Damage per shot 7 | 7
Dual shot stage. We have 2 sub-shots 4 | 3 | 4 | 3
Multi projectile stage. We have 3 projectiles per shot. AIM&Shoot | 2 1 1 AIM-Deflection&auto-shoot 1 1 1 | AIM&Shoot | 2 1 1 AIM-Deflection&auto-shoot 1 1 1


Damage numbers for (crit, non-flank) or (non-crit, flank) since they all add +4 damage.
Total Attack damage = 2*6 (doubled base weapon damage since we're using dual shot) + 2 (ammo.) +4 = 18

Multishot stage. We have 2 shots per multishot. Damage per shot 9 | 9
Dual shot stage. We have 2 sub-shots 5 | 4 | 5 | 4
Multi projectile stage. We have 3 projectiles per shot. AIM&Shoot | 2 2 1 AIM-Deflection&auto-shoot 2 1 1 | AIM&Shoot | 2 2 1 AIM-Deflection&auto-shoot 2 1 1


Damage numbers for crit & flank
Total Attack damage = 2*6 (doubled base weapon damage since we're using dual shot) + 2 (ammo.) +4 +4 = 22

Multishot stage. We have 2 shots per multishot. Damage per shot 11 | 11
Dual shot stage. We have 2 sub-shots 6 | 5 | 6 | 5
Multi projectile stage. We have 3 projectiles per shot. AIM&Shoot | 2 2 2 AIM-Deflection&auto-shoot 2 2 1 | AIM&Shoot | 2 2 2 AIM-Deflection&auto-shoot 2 2 1
(impossible to score 12 headshots in a row with a dual shot smg but see bellow )

So we have this 4 possibilities:
AIM&Shoot | 2 1 1 AIM-Deflection&auto-shoot 1 1 1 | AIM&Shoot | 2 1 1 AIM-Deflection&auto-shoot 1 1 1
AIM&Shoot | 2 2 1 AIM-Deflection&auto-shoot 2 1 1 | AIM&Shoot | 2 2 1 AIM-Deflection&auto-shoot 2 1 1
AIM&Shoot | 2 2 1 AIM-Deflection&auto-shoot 2 1 1 | AIM&Shoot | 2 2 1 AIM-Deflection&auto-shoot 2 1 1
AIM&Shoot | 2 2 2 AIM-Deflection&auto-shoot 2 2 1 | AIM&Shoot | 2 2 2 AIM-Deflection&auto-shoot 2 2 1

Depending of the individual trajectory/point of impact/situation for each projectile the game will pick the damage value (was projectile nr 8 headshot, was it also in the back ..etc)

Some +damage abilities can be activated out combat and it might sense to sometimes activate them before combat (especially) on missions without hard time limit. Some meta-knowlege(repeat mission) required since this pre-buffing has drawbacks:

WARCRY/Sally is an (at least 4 tile) AOE buff for 1 attack once per mission (might be all characters in Sally's direct line of sight regardless of distance).

PAYBACK costs at least 1 hitpoint per attack, unless triggered with limited invulnerability charges (with Piper's full heal and the large hitpoint pools of Seabass & Ivanski this can be used a lot of times).

Only BERSERK/Sally is a permanent buff, until death (which is typically 1 shot away since it costs so much health to activate and Sally has so little) or too much healing(Piper). The rounding for the 50% condition favors the player. (rounds up, so for example 4 remaining HP out of 7 will activate Berserker)

Best way to "hurt yourself" in a controled way out of combat is probably the oil grenade (-2 hitpoints per activation before +1 if Mend).
Friendly fire explosive weapons can work(? need the kind that does not bouce from cover) (very dangerous if multiprojectile).
Explosive barrels also work for this -1/-2/-3 damage at 1 tile gap, -?/-5/-? damage next to it which is not worth it.

- Oil patches from pipe leaks, oil barrels or oil grenades can be detonated with a single projectile (bullet) and will do 2 damage to all player units standing in it. (WHAT ABOUT ENEMIES ? DOES 5 damage to enemies). Oil grenade item has 3 charges per mission compared to explosive grenades that only have 1 and has huge AOE. (shooting targets in oil patch will not detonate it unless shot from bellow to cross the oiled area before hiting the target - you need to "waste" a projectile on an empty drenched tile)
- Can multi-projectile weapons setup and detonate it in the same shot? (No, oil leak from destroyed barrel spills after all projectiles expire & explosive containers detonate) (multishot weapon can set up oil barre & /detonate it & kill weakened survivors)
- Explosive barrels will do 5(the core area) damage to adjacent player units when they go off. They will typically do enough to kill most mook enemies in 2 tile radius.
- Sharpshooter can bounce a bullet of explosive cover (setting of the explosive and potentially killing victims near it) and also kill the intended bullet destination: single non-piercing projectile multi-kill)
- Stage Bosses (Chop Sue/Red Queen/Vectron) are immune to stun :( .The mooks and mini-bosses are not. This makes stun afliction (weapons/items) pretty useless since you can just 1 shot kill mooks /per action anyway even on elite difficulty. (why waste an action (trying) to stun a mook when you could just kill it instead. And against the stage bosses where the effect would actually be useful against - it has no effect ).
-there are no item stacking restrictions (ex can use the radioactive bullets twice :) for +4 damage, or same speed boots twice on the same character) Particularly intresting is stacking 2 mending items on Seabrass that already has mending (will heal up to 3 damage per turn. Mobility and damage items probably better, still interesting ) .
-weapons with "piercing character" and cover bouncing projectiles (ex Valentine's piercing shot) CANNOT damage the same target multiple times :( even if it is possible for this type of projectiles to hit the same character multiple times. ONLY EXCEPTION I'M AWARE OF IS "THE KILLING JOKE" near endgame mission reward. The ability to pierce characters helps its damage potential but also killing shield orbs protected behind shielded enemies. THIS WEAPON MIGHT BE THE ONLY WEAPON CAPABLE OF DAMAGING THE SAME TARGETs MULTIPLE TIMES WITH THE SAME PROJECTILE (AOE EXPLOSIONS WILL TRIGGER AND DO DAMAGE AGAIN WHEN HITING NEXT TARGET OR COVER BEHIND THE FIRST TARGET)
-it is possible, near the end of the game to have a multi-shot weapon achieve enough non-crit damage per shot to kill the weaker engame mooks on elite difficulty.
Example:
Valentine lvl 10 with Death Ray Tier 9 Rare long range Sharpshooer weapon available on one of the vendors. 12 base weapon damage + 2 lvl 9 Piper Inspire, + 3 Complete Focus lvl 10 Valentine, +3 Berserker's Ammo, +2 Radioactive ammo = 22 which will split 11 11 per shot. Nicer yet, it works with piercing shot.
Piper lvl 10 (inspired) can do the same with Power Shot (no char piercing).
Dora lvl 7 has an even easier time: 12 weapon base + 6 crit on weapon + 2 flank (3 backstab at lvl 10) + 2 radioactive ammo = 22 as long as she hits the targets in the back.