Editing Warhammer 40,000/9th Edition Tactics/Apocalypse (section)

==A guide to Hit and Wound rolls==
If you've gotten so far that you're reading a tactics page on the sewer drain that is 4chan then you should already know the basics of how dice rolls work, and how the warhammer wounding system works. To begin however, you need to appreciate that each dice roll is a probability of success:
*6+ is 16.66% chance
*5+ is 33.33% chance
*4+ is 50% chance
*3+ is 66.66% chance
*2+ is 83.33% chance

Eagle eyed or knowledgeable readers will have noticed/know that each jump represents a 16.66% increase. However, lets look at the improvements you gain with each level of dice roll, and by extension your potential damage increase.
*5+ is 100% better than 6+
*4+ is 50% better than 5+
*3+ is 33.33% better than 4+
*2+ is 25% better than 3+

As you can see, your returns for acquiring better hit/wound rolls diminish rapidly, which is why ork and tau overwatch is so dangerous, as they are effectively hitting on a 5+ on overwatch, a 100% increase over only getting overwatch on a 6. The same applies to if you automatically succeed a roll, the worse your normal roll would be, the better the improvement would be. This is also why armies like space marines don't deal that much more damage with their weapons over eg. Guard armies, as their improved ballistic skill only buys them 33% more damage, while costing more than that.

What you are getting from your improved rolls is not just output, but reliability. As you read through this page you can end up in the dangerous mindset that the numbers you read are absolutes. They are not. They are statistical likelihoods. They are averages, and averages are best on a range, a range which will come to bite you in the ass when you least expect it.

What better rolls buy you is reliability, which is exactly what you want when you're dropping a kill squad out of deep strike to anally abuse the enemy commander. Likewise, is it more cost effective for you to bring  lot of guns which wound on a 4+, or less guns which wound on a 3+? So lets have a look at the dice rolls again, but instead look at their fail rates:
*2+ is 16.66% fail chance
*3+ is 33.33% fail chance
*4+ is 50% fail chance
*5+ is 66.66% fail chance
*6+ is 83.33% fail chance

And then the improvements:
*2+ is 50% less fail than 3+
*3+ is 33.33% less fail than 4+
*4+ is 25% less fail than 5+
*5+ is 20% less fail than 6+

As you can see, a 2+, even though it doesn't buy you much more in the way of success, it buys you a big reduction in relative fail rates, which is exactly what you want when you are looking to guarantee a result that you want. On the opposite, the previous example of the tau and ork overwatch, they may have doubled their hit chance, but they have only reduced their fail chance by 20%, so most of their attacks are still missing.

===Rerolls===
Much has been said about rerolls. If you go to the separate tactics pages you will bump into the effectiveness of captains reroll auras, and whether they are worth it. First off, we need to have a look at the effect they have, both rerolling 1's, and rerolling all fails:
*6+ is 16.66% chance
*5+ is 33.33% chance
*4+ is 50% chance
*3+ is 66.66% chance
*2+ is 83.33% chance

Reroll 1's:
*6+ is 19.44%, +2.78%
*5+ is 38.88%, +5.55%
*4+ is 58.33%, +8.33%
*3+ is 77.77%, +11.11%
*2+ is 97.22%, +13.89

Reroll fails:
*6+ is 30.55%, +13.89%
*5+ is 55.55%, +22.22%
*4+ is 75%, +25%
*3+ is 88.88%, +22.22%
*2+ is 97.22%, +13.89%

To explain what you are seeing, rerolls are effected by two factors:
*Your roll chance
*And your reroll chance to fail ratio.
The higher these two values are, the better your roll will be. This is why when rerolling 1's, a 2+ roll sees the best increase, as it has the highest initial success chance, but it is also why a 4+ has the best increase when reroll all fails. So like with getting better basic dice rolls, when looking at rerolls you need to consider what that reroll is buying you. Rerolling 1's for orks won't buy you much (although in some builds that increase may be enough for you to have to buy bigger bucket for your dice), but rerolling 1's for custodes for their 2+ to hit sees them only failing their rolls 2.78% of the time.

However, rerolling also buys you something else, and that is avoiding negative effects of dice rolls. The most common effect like this is plasma, which deals mortal wounds or outright kills a model on a roll of 1. Rerolls bypass this by giving you one chance to take that 1, a reroll it. Lets take space marines for example. Plasma in their hands will hit 66.66% of the time, and kill them 16.66% of the time when overcharged. When rerolling (whether 1's or all fails) that 16.66% goes to 2.78%, or a 1 in 36 chance.

===Modifiers===
8th edition used to be the era of the modifier, with everything from elder minus 4 to hit cheese to necron Tesla guns activating on a 4+ to hit rather than just a 6 (because who doesn’t want to multiply their initial shots by 7/6).

9th has made everything a whole lot simpler (and less cheesy) by limiting modifiers to just plus or minus 1 once they are all added together. This means that you can still give a unit plus 2 to hit, all that means is that you’ll ignore a minus 1 to hit penalty and still get plus 1 to hit.

There are a few other important changes around modifiers:
*Effects which trigger on a 1 now only trigger on a natural 1, so weapons like plasma are a lot safer to use.
*Strength modifiers are now done after any effects created by a weapon, a S4 marine with a thunder hammer now hits at S9 instead of S10.
*A roll will always succeed on a 6, and always fail on a 1. The exception to this are armour saves, which are always failed on a 1 but can fail if the dice roll required would be higher than 6 (eg. 7+).

===Auto Hitting aka the Humble Flamer===
There’s nothing like purging the heretics with fire. Or destroying the corpse worshipers with an infernal inferno. Or dissolving the walking food with bio acid. Or crushing the deniers of the great plan with plasma wash.

Whatever form it comes in, flamers are everywhere. In previous editions (read, 7th and earlier) the flamer used a template for its shots, and you would roll to hit all units under it. As a result, hordes were really difficult to use, as your gaunt swarm could easily be destroyed by a guardsman with a basic flame thrower. New flamers are characterised by a few points:
*They have a random number of shots (eg. D3, D6, 2D6 etc).
*They automatically hit their target.
*They have shorter ranges than similar weapons.