Editing Warhammer 40,000/8th Edition Tactics (section)

===Combat 101===
While Shooting and Fighting are separated below, they largely share rules, much like how Movement and Charging are in separate phases, but share many rules and concepts. By and large, ''any'' attack you make has 5 values: Attacks (which is rate of fire), Accuracy (typically WS or BS), Strength, AP, and Damage. These values are converted into other values in context (for example, attacks are left alone, but accuracy requires dividing by 6), then those values are multiplied together to determine how many wounds you actually expect to knock off your target. They resolve in this order, in general:

#Determine number of Attacks.
#Roll to hit, based on accuracy.
#Roll to wound, based on strength and toughness.
#Target rolls to save, based on its saving throws.
#*While all rolls are subject to re-rolls and modifiers, the most common modifiers in the game apply here, such as AP and Cover.
#Roll to damage.
#If the target has a "Feel No Pain" type rule, it rolls that against damage, much like a saving throw. Feel No Pain rules cannot stack with each other- only the strongest one is applied. 

As will be discussed below, you ''always'' re-roll ''before'' applying modifiers.

====Attacks====
This can be random, such as 1d3, or 1d6, or 2d3. There is a section below on dice averages, but for attacks, you can safely treat a random value as its average for the purposes of working out how much damage a given attack will do to a given target. More often, it will be a constant number- melee weapons generally use the Attacks stat of the user, while ranged weapons will specify the number of attacks they can perform in their profile.

====Hitting====
By and large, this will be a WS or BS value. Regardless of the name of the stat, here called "AS" for "Accuracy Skill". 

The odds of hitting is: (7-AS-modifier)/6

*An ability to re-roll will multiply this value by (6+x)/6, where x is the number of facings on a hit roll you can re-roll, so if you re-roll 1s x is 1, re-roll 1s, 2s, 3s and 4s x is 4.
*Re-rolling all failed hits means x=7-AS, note that modifiers do not change the effect of re-rolling all failed hits, this is only affected by the AS. 
*Abaddon's ability to re-roll hit rolls even if they hit mean x=7-AS-modifier instead.

====Wounding====
Instead of requiring a fixed roll like hit rolls most wound rolls (WR) instead require that you compare the Strength of the weapon to the target's Toughness, although some weapons do require a fixed roll. 

Your WR is 2+ if S ≥ 2xT, 3+ if S > T, 4+ if S = T, 5+ if S < T, 6+ if 2xS ≤ T

The odds of wounding is: (7-WR-modifier)/6

*An ability to re-roll will multiply this value by (6+x)/6, where x is the number of facings on a wound roll you can re-roll, so if you re-roll 1s x is 1, re-roll 1s, 2s, 3s and 4s x is 4.
*Re-rolling all failed wounds means x=7-AS, note that modifiers do not change the effect of re-rolling all failed wounds, this is only affected by the WR.

====Penetrating Saves====
Basic saving throws work very intuitively, broadly identical to accuracy a 6+ save works just like 6+ to hit - except that the target assigns which model takes the save in the unit and rolls the saving throw, this changes the looks of the math a little since we will be calculating how likely we are to penetrate our opponent's save rather than how likely we are to save. Many weapons have a negative AP value that increases the dice roll your opponent needs to beat to pass their saves, a cover save now adds a +1 modifier to the Sv of the unit rather than providing an alternate save like an invulnerable save like it did in the past. A saving throw roll of 1 is always a failure, but a roll of 6 is not necessarily a success. Invulnerable saves are unaffected by cover and AP but otherwise works exactly the same way as a normal save.

Your odds of penetrating (oop) is (Sv - modifiers - (AP) - 1) / 6 

*Note that oop cannot be greater than 6/6.
*Note that since AP is negative (-1/-2...) the outcome of AP is actually positive -(-1)=+1. 
*An ability to re-roll will multiply this value by (36 * oop - x * (7 + Sv + modifiers + AP) ) / 36 * (oop) where x is the number of facings on a saving throw your opponent can re-roll, so if your opponent re-rolls 1s x is 1, re-roll 1s, 2s, 3s and 4s x is 4.
*Ap increases damage non-linearly, having AP- or AP-1 against a 2+ Sv and a 6+ Sv are very different things, against a 2+ Sv AP- will need 6 wounds to penetrate the save once, while AP-1 will need 3 wounds to penetrate the save once (causing 100% more damage), while against a 6+ Sv AP- will need 1,167 wounds to penetrate the save once, while AP-1 will need 1 wound to penetrate the save once (causing 16,7% more damage).
 
=====Cover=====
Cover improves the Sv of the unit that is in cover by 1 against Shooting attacks regardless of which phase that Shooting attack is made in or whether the target or firing unit is in combat. A model cannot claim multiple cover saves, it is either in cover and receives the bonus or it is not and it does not benefit. If you have a unit that is partially eligible for cover the whole unit will not benefit, but if you remove all the models from the unit that are not eligible the unit benefits immediately, therefore it can be beneficial to roll your saves one at a time and pick off the ones outside cover first. 

*Certain units and terrain types (in the advanced terrain rules) have special rules which require additional restrictions regarding what units count as in cover or not. The best way to negate this if you want to maximize your output is to spread Anti-Armour units out across the board for as many viewpoints as possible. As for actually using this type of cover yourself, your best bet is to park these units inside a building or in a covered corner of the board where the opponent can't try to maneuver into a better place to shoot.
**Most TITANIC units require you to have them '''''in''''' cover '''''and''''' be 50% or more obstructed from view from the shooter (and sometimes this simply doesn't apply to some models). This would found as a special rule of the TITANIC unit itself and is not, strictly speaking, a rule for all TITANIC models. 
**Woods require non-INFANTRY units to be '''''in''''' cover '''''and''''' be 50% or more obstructed from view from the shooter.
**Ruins require non-INFANTRY units to be '''''in''''' cover '''''and''''' be 50% or more obstructed from view from the shooter.
**Craters only provide cover to INFANTRY.
**Barricades require INFANTRY units to be within 1" and behind it from view from the shooter. They do not provide cover to non-infantry.
**Obstacles do not provide cover.
**Imperial Statuary provides cover to MODELS that are within 3" and 25% obscured from the point of view of the firer.
**Fuel Pipes require INFANTRY units to be within 1" and behind it from view from the shooter.  They do not provide cover to non-infantry.
**Battlescapes require non-INFANTRY units to be '''''in''''' cover '''''and''''' be 50% or more obstructed from view from the shooter.
**Hills do not provide cover.
*A special note that terrain obscuring requirements from the view of the shooter doesn't have to be obscured by the terrain feature itself. A Vehicle could be on a Battlescape terrain feature which did not conceal them, but if another terrain feature or model obscured at attacker's view of them, it would meet the requirement to be obscured from the view of the firer.
*Many of the terrain types have additional special rules, but not rules related to meeting the requirements for cover saves.

====Inflicting Damage====
When a model fails its saving throw it takes a number of damage depending on the Damage characteristic of the weapon used. The model suffers that many wounds and any excess wounds are lost. 

Feel No Pain "FNP" style abilities allow models to ignore some of the damage they take on by rolling a dice and beating a number, these rules are exclusive meaning you have to use the best one available but you can use it in addition to your normal saving throw.

Roll a die each time a model with this rule suffers a wound or mortal wound. On a roll of 6, ignore it.
*Sometimes, the roll is a 5.
*A 6+ FNP increases the average number of wounds you need to deal to a model to kill it by 6/5 or 20%. The chance that it will completely nullify an unsaved wound is 1/46656‬ for 6 damage, 1/7776 for 5 damage, 1/1296 for 4 damage, 1/216 for 3 damage, 1/36 for 2 damage and 1/6 for 1 damage. 
*A 5+ FNP increases the average number of wounds you need to deal to a model to kill it by 6/4 or 50%. The chance that it will completely nullify an unsaved wound is 1/729 for 6 damage, 1/243 for 5 damage, 1/81 for 4 damage, 1/27 for 3 damage, 1/9 for 2 damage and 1/3 for 1 damage.