There's a decent chance you are capable of shooting more enemy models dead per turn. There's a chance you've been giving enemies cowering in cover too high a save. There's a chance you can get your basic squads to operate more like Snipers. And there's a chance you can find ways to shoot enemy models without causing your assault on their squad to fail.
After seeing a debate about Cover Saves in the 3++ chatbox yesterday last for more than two hours, I thought it worth taking a stab at the rules. Most of the site's visitors are the more competitive, tournament-orientated 40Kers, so if we're one year in to the 6th Edition Rules and there is still that level of confusion, it must be worth an article.
Half of this article is very much a back to basics article, but it should include at least something to help gamers of all levels kill targets more efficiently.

The Basics

At it's most basic level, cover saves are done on a model by model basis, rather than unit by unit.
DETERMINING COVER SAVES
If, when you come to allocate a Wound, the target model's body (as defined on page 8) is at least 25% obscured from the point of view of at least one firer, Wounds allocated to that model receive a cover save.
What saves do you get?aegiscalm
  • Razor wire 6+
  • Intervening Models 5+
  • Gaps between Intervening Models 5+
  • Forests and area terrain 5+(Vehicles that explode are replaced with Area Terrain (5+ cover) page 74 or a crater 5+ cover)
  • Ruins, ruined fortifications and trenches 4+
  • Fortifications 3+
  • Aegis Wall 4+
What bonuses does Going to Ground give?
  • Go to ground behind Aegis 2+
  • Go to ground adds 1 to cover save (6+ in the open) Page 18
  • Go to ground in area terrain adds 2 to the model's cover save Page 18
Nothing note-worthy yet, right? Well actually no, we've already passed one of the most important pieces of information about cover, one that was misunderstood by several people in the chatbox conversation and one that I very frequently hear a tournaments.

The Myth of 2+ Area Ruins.

Area Terrain is always difficult and always gives 5+ cover regardless of whether models are 25% obscured or not (page 91), with the exception of Vehicles, which are not obscured by Area Terrain and must be 25% obscured from the point of view of the firer. (page 75)
The key is that Ruins are not Area Terrain, and Area Terrain is not ruins. They have seperate entries on the Cover chart, but it is extremely common for players to class terrain as ‘Area ruins' or say ‘This ruin is area' while setting up, and then get 2+ when going to ground in it.
If it's a ruin and the model goes to ground, it gets a 3+ save (one better than an a ruin's 4+). If it's area terrain and the model goes to ground, it gets a 3+ save (two better than area terrain's 5+).
Either way, the best save without stealth, shrouded or other special rules will be a 3+ save. If you're one of the many players who have been giving opponents 2+ saves, feel free to kill them faster.

What if it’s both Area and a Ruin?

Area terrain should have a base that shows its boundary (page 91), and it is true it is reasonably common to see area terrain on a base with a clear boundary that contains ruins and rubble, and could be considered a ruin. GW acknowledge this in the rules (Page 98) :
“Ruins with Bases
A ruin might be mounted on a base, decorated with rubble, and other debris. In this case, treat the base as area terrain.”
That means a model on that base is in area terrain, and gets a 5+ save. It could GTG and get a bonus of 2 for a 3+ save. If the base contains enough walls and rubble to obscure 25% of the model as in the illustration below, then the player could instead GTG and get a bonus of 1 to the Ruin's cover save. In either case the result is the same and the model gets a 3+ cover save.
gotoground
Killer Tip 1: In neither case can the model gain a 2+ save. The model gets a 4+ save from Ruins with a GTG bonus of 1, or an area terrain 5+ save with a GTG bonus of 2.
It is still important to define whether this sort of terrain piece is Area or Ruins however, since models from the smallest ratlings to the largest Monstrous Creatures gain cover from having a tippee-toe in Area Terrain, but need to be 25% obscured to gain cover from ruins.

Focus Fire

While the above covered one of the most misunderstood basic rules about cover, the following will cover one of the most overlooked advanced rules.
Focus Fire (page 18)
“Sometimes, a target unit will only be partially in cover, with some of its models in cover and some not. In this case, you have a choice: your unit can either shoot at the squad as a whole, or you can declare that they will Focus Fire on the enemies who are less hidden. If you choose to Focus Fire, you must state your intention before making any rolls To Hit.
If you choose to Focus Fire, choose a cover save value. This can be between 2+ and 5+. Your opponent can only allocate Wounds to models with a cover save equal to or worse (i.e a higher value) than the value stated. “
I consider this rule so powerful that it is a potential game winner for a shooting-orientated army, and yet it doesn't seem to get used all that much. I'll give you a couple examples of it from the final rounds at the last tournament I went to.
In the first example, a Tyrannid player fired 16 shots into a unit on my objective. The closest two models were behind an Aegis wall, and the rest in the open from the firer's perspective. In a friendly game I'd have suggested focusing fire on models in the open, but this being a tournament I let them try their luck. After ground to ground for a 2+ save, only a single model died, rather than the 8 who would have been almost certainly dead to fire that was focussed on models with no cover save.
In the following game, I was up against Tau with the immortal Iridium Commander leading Crisis Suits. My opponent had the Iridum Commander and two other suits touching area terrain, but the other two behind them were in the open. Seizing the opportunity, I moved a PBS into range and focus-fired plasmaguns at the rear Crisis Suits, bypassing the Commander and his fancy saves and rules, and caused two casualties. Casting Weaken Resolve on them meant the Commander and the remaining suits fled off the table.
Both of those examples were game-changers, but this sort of situation comes up in most games if you look for it (although unfortunately this particular Tau opponent has kept his suits all in equal cover in every game since then).
Here's a third example using a photo from a game at my house last week. The Librarian (who I will admit looks suspiciously like Darth Vader) is attached to a squad of Marines. Unlike the example with Tau above, most of the time the Independent Character is the best target in the squad, so rather than using the Focus Fire rule to avoid putting wounds onto the IC they can be used to put them onto him. The fact he is at the back of the squad is irrelevant – using Focus Fire can turn any unit into snipers.
FOCUS
By declaring Focus Fire against models with 6+ or worse cover, the Dakkajet's shots will all go into the librarian, as the 5 black and red marines are all in or touching area terrain. Rather than being wasted on cheap MEQ ablative wounds, the shots can go straight in to the Sith Lord himself.
From even this simple example there are three more things to keep in mind
  1. Should all models with this level (or worse) cover save be killed, any remaining wounds are lost. “Your opponent can only allocate Wounds to models with a cover save equal to or worse (i.e a higher value) than the value stated”. That is a downside, particularly when firing as many high strength shots as that Dakkajet will.
  2. Characters can still use “Look out Sir!” to bounce wounds on to other models in their squad, regardless of their cover save. That makes Focus Fire more useful when trying to avoid characters like Iridium Commanders, Draigo and similar than when trying to kill them, except:
  3. In this case, the closest marine to the character has a lascannon, and the second closest is the sergeant. If the player is unlucky with LoS rolls, the Librarian may die. If the player is lucky with the LoS rolls, the Lascannon and Sergeant will probably die instead of cheap chumps. If the player is really unlucky, they will bounce enough wounds to kill the lascannon and then keep enough on the Librarian to kill him too. That happens more often than you'd think.
Killer Tip 2: Start using Focus Fire. Use it every time it won't result in over-kill, even when you don't need to; just get into the habit.
Killer Tip 3: You can create cover saves for your enemy's models. Just as experienced players block their own unit's line of sight using vehicles in order to prevent shots going into undesirable targets, you can also use your own intervening models from other squads to give cover saves to some models in enemy units – for example, using Grey Hunters to give a Tau Iridium Commander a 5+ save from your Long Fangs means with focus fire all the missiles will go in to the Crisis Suits behind him.
And here's one to bring a tear to the eye of general's commanding less shooty-armies.
Killer Tip 4: Use Focus Fire to shoot your pistols at low cover save models at the back of enemy units before you charge. You can kill enemies without shooting yourself out of range and causing your assaults to fail!

Now, get out there and kill something.

-Matt