Cover rules are garbage. For instance:
Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.
There are three degrees of cover. If a target is behind multiple sources of cover, only the most protective degree o f cover applies; the degrees aren’t added together. For example, if a target is behind a creature that gives half cover and a tree trunk that gives three-quarters cover, the target has three-quarters cover.
A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.
A target with three-quarters cover has a +5 bonus to AC and Dexterity saving throws. A target has three-quarters cover if about three-quarters of it is covered by an obstacle. The obstacle might be a portcullis, an arrow slit, or a thick tree trunk.
A target with total cover can’t be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.
That’s from the 5e rulebook, which mercifully does away with D&D 3e’s nonsense. In case you weren’t aware, 3e differentiated between partial cover, total cover, soft cover, improved cover, and then they added special rules for concealment onto it. Adding an additional roll to every attack checking to see if you hit somebody’s Armor Class but then actually missed because it was foggy? Sure, why not.
Anyway, here’s how I’m running cover in all my games forevermore.
If you’re behind full cover, you’re ducking, crouching, or standing still behind something that makes it hard for enemies to hit you. When you’re taking cover, you can’t be hit straight-on. If somebody shoots at you, he’s going to hit the cover instead of hitting you. There’s no dice rolling. Two caveats to this.
1. Enemies can circumvent cover by moving around to get a better shot at you. If they do so, the cover doesn’t do a whole lot of good. On the flip side, you may need to move around to get into a better location so you can shoot at an enemy behind cover.
2. If your cover gets worn down from enemy gunfire (think that scene in the original Matrix where they shoot up the lobby and the pillars are whittled down), you can no longer take cover. You either move or you get shot. Likewise, you can shoot at the cover someone’s using to wear it down, but it’s only getting destroyed if you’re bringing heavy enough firepower.
If you’re not taking cover, you might still benefit from partial cover. That’s when attacking you is difficult because there’s a bunch of stuff in the way. This includes anything from you being in melee with another dude, you poking your head out from a barricade, or you sniping from the bushes. This assesses whatever bonus/penalty is most appropriate: if you’re playing 5e, you get advantage on Dexterity saves, or maybe the orc shooting at you takes disadvantage on his attack roll. In Savage Worlds, it’s a -2/+2 because tracking more than that is annoying.
Some additional thoughts on how you might do cover in Fate Core specifically:
I hate the concept of rolling Athletics to Create An Advantage with cover. That seems like a bunch of bullshit to me. If you’re behind a wall, you’re behind a wall, whether or not you succeed on your roll to generate a free invoke. Oh, I know, you’re rolling to see if you can make it narratively important–but again, that seems like a bunch of bullshit to me. Why even bother rolling? (Actually, there a whole lot of things in Fate Core where the dice are completely superfluous, and that’s kind of pissing me off, which is why I’m thinking about running it as a 1d6-1d6 system like Legends of Anglerre, but that’s another rant for another time.)
1. You can pay a Fate point to get yourself a +2 bonus from cover as normal.
2. When you get hit by an attack, you can reduce the damage of the attack to 1 stress, but you can no longer use the cover. It’s destroyed, you’re flushed out, or something else prevents you from utilizing it.
3. Cover grants you Armor: 1.