The Care and Feeding of Warriors Armor Penetration I

Armor Penetration has been with us in one form or another for quite a while now. There are abilities like Sunder Armor and Expose Armor that lower armor temporarily, of course, and the rogue talent Serrated Blades. My first conscious exposure to the mechanic was the epic weapon Bonereaver's Edge, which dropped off of Ragnaros. Back then, the mechanic was fairly simple. Bonereaver`s Edge would ignore a certain amount of armor with each proc of an on-hit ability, in this case 700 armor. It could stack up to three times, so in a fight that lasted for long enough Bonereaver`s could maintain an effective -2100 armor debuff on a boss that only applied to the person using it.

Effects like this weren`t terribly common in Vanilla WoW. I myself never had a Bonereaver's (Don't cry for me, I did all right on Rag drops if I do constantly brag so myself) and so Armor Pen didn't really impinge on my consciousness. Of course, I was mostly either a tank or an offtank back in the old MC/BWL/AQ/NAXX40 days anyway. Back when you could tank with an arms or fury spec and dinosaurs ruled Un'Goro. (They still do, we just don't go there very often.) So it wasn't until Burning Crusade that I really started to notice ArP.

Back in BC, armor pen didn't have rating yet. Enchants like Executioner read "Permanently enchant a Melee Weapon to occasionally ignore 840 of your enemy's armor. Requires a level 60 or higher item." Gear that had armor pen on it told you how much armor it was going to penetrate. Cataclysm's Edge, for instance, just said "Equip: Your attacks ignore 335 of your opponent's armor." What this meant was, when you collected a whole set of ArP gear, all you had to do was add up how much armor you were ignoring. The plus side of this was, it was very simple to understand. The down side? Well, on bosses or classes with low armor (we're talking those annoying skirt wearers who can take half of your health off in one attack that completely ignores armor, you know the ones) reducing up to, say, 3000 armor at level 70 was pretty dang nasty. So they changed Armor Pen to a rating.

From there, all our troubles began.
Armor Pen as a rating has been fraught with difficulties since Wrath launched. The move to a rating based system and to percentage based ArP (ignores a percentage of target armor rather than just X amount of target armor) was aimed at retaining or even enhancing the stats usefulness against high armor targets while making it less ridiculously lethal against low armor targets. Rather than easily stacking enough ArP to rip a shadow priest's head off while barely even scratching a Bear Druid or Protection Paladin, you'll have the same boost to your damage against each target regardless of their personal armor. The change to a rating was also aimed at falling in line with other stats like hit and crit which require more and more rating to achieve the same results as a character levels.

As a result, between the launch of Wrath and patch 3.1, nobody wanted Armor Penetration on their gear. It was so bad that when I noticed the sheer amount of ArP on Ulduar gear and what that meant for how much armor you could actually ignore, especially with certain Arms talents and chances to Battle Stance, it was (to put it mildly) not well received. The buff to Armor Penetration's effectiveness is what ultimately led me to this conclusion: However, it soon became clear that Armor Penetration, while still a somewhat underwhelming stat for classes that do a large percentage of their damage as magical, had become fairly compelling for warriors as a DPS stat. Initial reports that ArP could in fact negatively reduce target armor at a certain point prompted the stat to be capped so that it could no longer reduce armor below 0, meaning that roughly 1399 Armor Penetration Rating = target armor reduced as low as it is possible to go in the game. In essence, if you had 1000 ArP rating and a Grim Toll equipped, which when it procced would push your ArP rating to 1612 ArP rating, you would be wasting about 200 of the proc's ArP because it's impossible to push your target below 0 armor. This doesn't mean that you will automatically push your target to 0 armor with that much ArP, however, as the 'up to' in the tooltip is something you see pointed out a lot.

I've not found a really coherent listing of how they've changed ratings in 3.2.2 at all. If you have one that takes the new values into account please feel free to post it here for discussion. The best source I have is this Elitist Jerks thread. Taking its values, we're currently looking at 13.99 ArP Rating to reach 1% armor reduction.

If you really, really love math, you can pour over the formula for Armor Penetration, which will allow you to find all the ways in which I've just messed up that example. To allow Ghostcrawler to explain in his own words:

• We didn't want Armor Penetration Rating to be too powerful against low armor targets, like it had been in BC. We also didn't want Armor Penetration Rating to be too powerful against high armor targets.
• So, we decided on a system where there is a cap on how much armor the Armor Penetration Rating can be applied to. So, the first X armor on the target is reduced by the percentage listed in the Armor Penetration Rating tooltip, and all armor past that X is unaffected. Another way of understanding that is we multiply the percentage in the tooltip times the minimum of the two values: the cap, and the amount of armor on the target after all other modifiers.
• Computing the cap is a little tricky unless you are already familiar with how World of Warcraft armor works. There is an armor constant we'll call C. C is derived as follows (in some pseudocode):

If (level<60)
 C=400+85*targetlevel
Else
 C=400+85*targetlevel+4.5*85*(targetlevel-59);

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