The Astral Aim rule on the Grey Knight Purgation squad has caused no end of debates around my neck of the woods. I am very appreciative of the Grey Knight FAQ which was recently issued but noted that this issue was not addressed. It may seem silly, but please note that this has caused some heated arguments and is, I think, worthy of a FAQ.
Specifically, the questions arise around units which would benefit from cover saves regardless of the presence of absence of the Astral Aim cover save. Specifically the question is whether and when a unit with a cover save which is BETTER than 4+ can use that cover save instead of the Astral Aim 4+ save (for example Stealth or Pinned units that are already in terrain).
The below is something of a dissertation, so feel free to not read it unless you are interested in the topic or just like rules lawyering. That being said, I do think it's quite a well written and researched summary, so I would encourage you to continue.
The 5h Edition 40k rulebook contains an FAQ which some interpret as directly contradicting the core rules as written. The FAQ is as follows:
Q: If a unit is in multiple different types of cover, which cover save is used? (p22)
A: Whichever has the highest number of models in, or the best in a tie (as long as over half the unit is in some kind of cover).
Now, some interpret this as contradicting the following excerpt from page 24 of the rulebook:
"If a unit can benefit from different types of cover, for example being behind a hedge (5+ cover) and a low wall (4+), the unit uses the best cover save available (in this case 4+)."
Now, one possibility is that the FAQ is in fact directly contradicting the core rules. This is not beyond the realm of possibility, as we have seen in the past.
However once one realizes that "cover" and "cover saves" are two separate (albeit closely related) concepts, one can see that the FAQ does not contradict the core rules, if interpreted correctly. Indeed, if interpreted in light of the distinction between "cover" and "cover saves" one can see that the FAQ does not contradict the rules but actually clarifies a point which was unclear in the basic rules (which is, after all, the point of an FAQ), but does not overrule them in any way.
How do we know that "cover" and "cover saves" are two different things? Why, the rulebook tells us, of course. Page 21 states "...models that are in or behind cover receive a cover saving throw." Hence we know that being "in" or "behind" cover grants a cover saving throw, but cover and a cover save are not the same thing (one being a tree, the other being a dice roll).
Moving on, we can see that a situation might arise wherein, for example, an eight model unit finds itself being shot at. From the point of view of the firer, three of the models are behind a hedge (5+ cover save) and two of the models are behind a low wall (4+ cover save). This is a tricky situation not clearly covered by the core rule.
Individual models benefit from different types of cover, but the unit does not benefit from either cover specifically because the unit does not have a majority of models behind either form of cover. But the rules do say that (Pg 22):
"If half or more of the models in the target unit are in cover, then the unit is deemed to be in cover and all of its models may take cover saves." (Again, note the distinction between cover and cover saves).
And we know that a majority of the models are in some kind of cover, correct? So what to do?
Well, this is where the FAQ comes in, and neatly solves this situation. Let's look at the FAQ again:
"Q: If a unit is in multiple different types of cover, which cover save is used? (p22)
A: Whichever has the highest number of models in, or the best in a tie (as long as over half the unit is in some kind of cover)."
So in the above described scenario, we are no longer at an impasse. We know that half of the unit is in some kind of cover, and we know that the highest number of models are behind a hedge. Thus the unit gains a 5+ cover save. Please note AGAIN that the FAQ draws a distinction between cover and a cover save! Cause (cover) and effect (cover save).
Again, please note that this does not contradict the core rules in any way. Let us think of another scenario in which a unit of six infantry was standing in a crop field (which grants a 5+ cover save) and is also behind a low wall (4+ save). Every model is both in cover from the field and in cover from the wall. Thus they choose the best save available to them, which is the 4+ cover save granted by being behind a wall.
Now we come to the heart of the matter, Astral Aim. Astral Aim's special rule states
"The target automatically has a 4+ cover save (which cannot be modified by any means) against this attack."
Interesting! So clearly the target is entitled to a cover save of some kind, that much is clear! Also, it seems clear that this particular cover save (granted by the Astral Aim rule) cannot be modified. So no going to ground, no stealth benefit. Pretty cut and dry.
But please note that the rule does not place the unit in cover. It does not magically drop them into a forest. It does not improbably vault them behind a wall. It grants them a save. Saying otherwise would be like saying that the "Force Dome" special power (which grants a 5+ invulnerable save) causes the affected unit to be equipped suddenly equipped with a Crux Terminatus. Storm Shields grant a 3+ invulnerable save. Not everyone with a 3+ invulnerable save has a Storm Shield.
Horse dead yet? Beaten enough?
So let us conjecture that the target unit has a majority of it's models behind a stone wall. Those models are IN COVER. Because a majority of those models (let's say three out of five) are inside a forest, the unit benefits from a 4+ COVER SAVE. This is a separate cover save from the one granted by Astral Aim. The target now has TWO cover saves, but is only in ONE type of cover - a forest.
Now, let's say that the Purgation squad well and truly hammers the target, hitting with 9 shots and inflicting 7 wounds.
At that point (per page 24): "After the enemy has rolled to hit and to wound, but before you make any saves or remove any models, you can declare that the unit is going to ground... Models in a unit that has gone to ground immediately receive +1 to their cover saving throws."
So the unit receives +1 to their saving throws. Astral Aim cannot benefit from this modification, per the Astral Aim rules. However, the 4+ cover save granted from the forest WOULD benefit from this +1, meaning that this would be a 3+ save.
One could argue that before they make the "saving throw" (and thus receive the +1) both saves are equal (both being 4+). However the rule clearly states that the target "always has the advantage of using the best available save." Not the lowest save, but the best save. In this case the "best available" save is the one that allows for the 3+ gone to ground bonus, clearly. To argue that a save that will remain a 4+ is "better" than a save that will be a 3+ would require a feat of mental dexterity worthy of a public defender.
Indeed, as the term "saving throw" and "save" are used indistinguishably throughout the rules from my reading (such as on page 21), with "cover save" being shorthand for "cover saving throw" it seems unclear that the save itself is not immediately modified to 3+ (as would the other cover save were it not for the Astral Aim special rule) even before the roll is made, making the "best" and "lowest" functionally identical.
Either way, it seems plain that the target would be entitled to their 3+ "Gone to Ground" save from being in a forest.
Feel free to respond with your agreements, disagreements, or lamentations that I wasted time writing up such a silly thing instead of posting a BatRep.
