Trueshot Aura

The Theory

As every marksmanship hunter knows, the majority of the damage we deal is physical damage. This damage is mitigated through the armor that is worn by every player and creature in the game. Blizzard have given us a new toy to play with, Armor Penetration. Which will let us ignore a percentage of the enemies armor.
The theory here is to minimise the enemies mitigation, and as such raise the damage on the physical damage shots that we fire.
Physical Damage: Auto Shot, Aimed Shot, Silencing Shot, and Steady Shot
Magical Damage: Chimera Shot, Serpent Sting, Arcane Shot.
The top 3 of these in any boss fight are of course, Autoshot, Steady shot, and Chimera Shot. Usually in this order.
Between the 4 Physical damage shots we have roughly 60% of our total damage. Anything that applies to all of these shots can’t be bad right ?

The Math

To understand how ArP works first we need to understand how armor works.
We know the armor equations, Fairly common knowledge, for anything above level 60:

DR% = Armor / (Armor + 400 + 85 * (AttackerLevel + 4.5 * (AttackerLevel – 59)))
Lets plug in lvl 83 (A raid boss)
DR% = Armor / (Armor + 16635)
We can clearly see a diminishing returns model here. For every thousand armor, the damage reduction gained is reduced. The first 5000 armor gives 23.11% reduction, to double that we need 14300 Armor.

What does this mean for ArP?
We aren’t subject to any diminishing returns based on armor, we will always ignore by percentage. Taking the diminishing returns on armor into account this means that actually the higher our armor penetration, the more effective it is.
Take our example above for example. If we go from 0% to 65% armor penetration, we remove 23.11% of the bosses damage reduction. However, to gain the same effect again it would only take 35% armor penetration.

Lets get some real numbers here.
Ghostcrawler announced the formula for armor penetration before, Lets put all out formulas together here, and see if we can get a formula for the damage increase we should see on physical shots, for a given value of armor penetration
C = 400+85*targetlevel+4.5*85*(targetlevel-59)
We also know how much armor is succeptible to penetration:
(Armor + C)/3
Knowing that level 83 bosses have 13083 armor we can then plug this number in to give as a value 9906
The next step taken by the game is to compare armor values, To take the maximum succeptible value, or the bosses armor value. To see which is actually used. (i.e. if the boss only has 500 armor, we can’t ignore 2000)
The lowest value here is 9906 for level 83 bosses.

But wait, We forgot something: Raid debuffs, Sunder armor applies 20% armor reduction, and Faerie Fire applies 5%. Any good raid group will have these two debuffs (or others with the same value) up constantly. If we redo our above equations we get a boss armor value of 9812.5 and an effective penetrable armor value of 8815.75
Assuming we have hit the softcap (51% – gives us 100% with trinket procs) we can work out the increase this gives to our shots with and without trinket proc.

First lets apply our penetration.
9812.5 armor – (51% of 8815.75) = 5316.5 Armor
and with proc
9812.5 armor – 8815.75 penetrable = 996.75 Armor

Now, Lets use these values, and find out how much of our damage the boss is mitigating each time:
%Reduction = (Armor / ([467.5 * Enemy_Level] + Armor – 22167.5)) * 100
Plugging in our numbers gives the following
Before ArP (Full Armor) = 37.10% Damage Reduction
Passive ArP (No Proc) = 24.22% Damage Reduction
Trinket ARP (With Proc) = 5.65% Damage Reduction

This means without ArP only 62.9% of our physical damage is effective.
With passive, 75.78% (which is a 20.47% increase from having none)
With trinket proc 94.35% (or exactly a 50% increase in damage from having none)

If we account for the trinket uptime we can even break this to a general number for how much this is worth.
Typical logs suggest between 17% and 23% uptime on boss fights, We shall take a half-way value for this of 20%.
20% time with trinket, and 80% time with just passive gives us a 26.38% total increase to all our physical shots.
All this maths is done accounting for both raid debuffs on the bosses, And if the bosses have higher armor due to these buffs being missing, The damage you gain will become even higher.

How does this apply overall ? Assuming a 10.5 second rotation, roughly 65-75% of our damage is physical, if not more. (steady shot, autoshot, aimed shot, silencing shot), which means this is roughly a 19.5% damage increase over our usual damage without arp.
If you consider that a good portion of the ArP we will have before trying to build it up, this is a very high amount of damage to pull out of one “b-rate” stat.
Once you reach hard-mode ulduar, or ToTC Raid levels of gear, This damage increase is much higher than you will get from gearing without it, and once you have around half of this, even gemming for agility is no match!

Remember though, Being over the cap provides only a minor increase, as it will only apply when the trinket is not up, Anything beyond cap you want to gem for agility!

Note: The 10.5 second rotation I name above is the following:
Steady Steady Steady Steady Steady Chimera Aimed Then repeat
This rotation assumes you will weave in your silencing shot when it is off cooldown as it is not linked to GCD, and uses the Steady, Serpent, and Kill shot glyphs.

Due to latency, or movement you may end up dropping one steady from this, This is better than clipping chimera and aimed.

The “guide” Part

Use a 10.5 second rotation
Only gear for it when you have high stats beforehand and can reach the softcap (620 with grim toll, 567 with Mjollnir)
Use the glyphs listed above.
Use Imp Steady shot in your talent tree
and don’t even THINK of touching arcane shot if you’re standing still.