How To Get Your Pussy The Right Amount of Penetration
Since 3.1 Armor Penetration as a stat has been very attractive to feral druids. It is, after hit and expertise the number one tanking threat stat, and for cats, it greatly increases shred, mangle, FB, and melee damage. If it's so good, why don't all cats just go for pure ArP?!
The answer lies in the way armor scales and the way ArP caps. Armor it's self is a great defensive stat for tanks, but bosses also have a specific value for armor. Armor, the stat, suffers from diminishing returns. This means that the more armor a tank stacks, the less damage reduction each new point of armor will grant. Therefore, armor begins to be less valuable the more you have.
Armor Penetration does exactly the opposite of armor. Each point of ArP will reduce the armor value of the target by a percentage. We know that armor gets less valuable the more you have, so armor penetration benefits from exponential returns. This means that for each point of ArP, a larger and larger amount of non-bleed physical damage is able to penetrate the target's armor. In other words Armor Penetration gets better and better the more you have.
The cap for ArP is about 1400 rating. At this point all of the target's armor that can be ignored, will be ignored. The target will still have armor, but adding more ArP will not change that (ArP used to be able to work past 100% but it was hot fixed because classes like feral druids were seeing dps much higher than anyone expected).
So, when should cats begin using ArP? Well, because ArP gets better the more you have, when you only have a little, its not all its cracked up to be. Since the general non-raiding casual players have access to triumph badge gear and the new ICC 5 mans, its easier than ever to get to the point where ArP is useful. This is, for most, the soft cap.
Like other stats, ArP has a soft cap, and a hard cap. The hard cap, as stated above is 1400, which can only be achieved by those running 25 man ICC or 10 man ICC hard modes. The soft cap however, is dependent on a proc from a trinket. There are 3 trinkets currently in the game that can be used to soft cap ArP. They are the [Grim Toll] (Nax 25), the [Mjolnir Runestone] (10m Ulduar Hard Mode), and the [Needle-Encrusted Scorpion] (Heroic Forge of Souls). When the trinket is proc'd and the ArP on your gear/gems reaches 1400, you are soft capped. This happens at 735 for the Grim Toll, 788 for the Mjolnir Runestone, or 722 for the Needle-Encrusted Scorpion.
Does all of this seem a little too... "mathy?" Well, I enjoy it, but if you said yes, blizzard agrees with you. In Cataclysm ArP will be gone.
The guide talks about gemming around ArP so check that out if you're still unsure. In the end the question of armor penetration comes down to a cat-by-cat basis. While the guide goes over some simple rules for itemizing properly, Toskk's and Rawr are more specific tools that can be helpful.
Happy Shredding!
