One of the most common questions that you'll hear asked by Marksmanship (MM) Hunters - as well as Warriors, Rogues and Kitty Druids - is "when should I start to gem for armor penetration?" The simple answer is, as soon as you can reach the hard or soft cap through gems, enchants and food. The problem is the simple answer is not always the best solution.

Armor Penetration - often abbreviated as ArP - indicates the percentage of armor that will be ignored by an attack. ArP and damage scale together, the more ArP you have, the more potential damage you'll do.

For MM Hunters, ArP becomes more valuable once you are able to reach either the soft ArP cap or the hard ArP cap. For some, the goal is to reach 100% ArP either through the hard or soft cap. There is no point in exceeding 100% as any ArP over 100% will be wasted and provide no DPS benefit.

To reach the ArP hard cap you need to have 1,400 ArP rating constantly on your gear - in other words you must be at 1,400 without the help of trinket procs.

To reach the ArP soft cap, you need to be able to reach 1,400 ArP rating when your trinkets proc. The formula to calculate the soft cap is:

Soft cap = 1400 - Item Proc

This can be a little confusing if you haven't seen it before so here's a basic example.

You're a MM Hunter with passive ArP (ie. ArP from your gear and gems) rating of 722. At this point you still need 678 to reach the ArP cap. You equip the Needle-Encrusted Scorpion from the Heroic Forge of Souls. When the Needle-Encrusted Scorpion procs it increases your armor penetration rating by 678 for 10 sec. So each time it procs you reach the ArP cap and you will see your DPS spike. So in this case you can say that the soft ArP cap is 722 - in other words with passive ArP of 722 you will reach the cap each time your trinket procs.

Ideally you would like to be at 100% ArP all the time, but depending on the gear you have access to, this may not be possible. For most people however, reaching the soft cap is realistically possible. The problem is that depending on your gear, reaching the soft cap and running with an 'on proc' trinket may actually be a DPS loss compared to stacking 'standard' stats like agility and attack power, depending on your mix of gear.

Lets consider the Needle-Encrusted Scorpion again. The trinket has an internal cooldown of 45 seconds. When it procs, the benefit lasts for 10 seconds. The trinket has a 10% chance to proc on critical strikes. Depending on your critical strike rating, the trinket may take 10-20 seconds to proc once its internal cooldown is complete. Even if the trinket procs within 5 seconds of the cooldown finishing, this would mean that you are only getting the benefit of being at the ArP cap for 10 seconds of every minute. Considering that you have no control of when this 10 seconds window will occur, it could go to waste. If you are unlucky you could find yourself running to avoid a void zone or kiting an ooze when your magic ArP window opens. That would not be good.

Personally, on my MM Hunter my ArP is hovering around the 600 mark, and that's even with the massive passive 155 ArP I get from Deathbringer's Will. With four pieces of T10 gear, no matter how I gem and enchant I cannot get close to the hard cap. I've experimented running at the soft cap, and equipping an 'on proc' trinket, however in my case it proved to be a significant DPS loss. Long story short, there's no quick and easy answer as to whether or not ArP is the best option for you. If you can reach the hard cap without significantly reducing your other key stats then it's more than likely worth it. If you can reach the soft cap, whether or not chasing ArP is a DPS gain for you will largely depend on your other gear.