Guide is currently under construction.

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I'm a diamond 5 player currently experimenting with gangplank builds. His ultimate allows him to zone enemies from objectives or fights while doing tons of damage. Now that gangplank has barrels he will be able to zone even better. As a 5th item on gangplank you can opt to grab a liandry's torment to do some insane AOE damages.

CDR is very important on Gangplank because his damage comes from his barrels and ultimate. His barrels have a long charge time even at max level and his ultimate has a high base cooldown. By getting max CDR you can use gangplank's kit more effectively.

Armor pen runes will allow gangplank to do alot more damage mid to late game because his barrels ignore 60% of armor. Flat penetration is calculated after percent reduction. Having armor pen runes makes gangplank's early game extremely weak, but you can still farm relatively safely with q and zone with barrels since they hurt even at lvl 1.

Standard AD 21/9/0 mastery with 5% CDR instead of attackspeed.

CORE:
With just hydra and Tri-force as a core you will do tons of damage.

Gangplanks core 2 items are hydra Tri-force. You can opt to farm more by rushing avarice blade somewhere in your build for a GB later.

Ghostblade is better than last whisper because barrels already ignore 60% of target's armor. Last whisper's 35% armor pen will be calculated multiplicatively with the barrel's ignore armor resulting in 74% armor penetration. (100%-((100%-35%)*(100%-60%)))= 74%
The 20 Flat armor pen from ghostblade will be better until targets have approximately 150 armor.
150 armor * (100%-26%) = 39 armor
VS.
150 armor * (100%-60%) - 20 flat pen = 40 armor
In the rare case where the entire enemy team consist of tanks with 150+ armor, ghostblade will be the superior option.

Ghostblade's armor pen efficiency isn't the only reason why it is better than last whisper. It allows GP to be capped at CDR and additional crit. Crit chance will make gp's barrel hurt more. Gangplank is all about being farmed late game and it is quite easy for you to farm tons of gold using parley and AOE barrels to plunder gold. This means slot efficiency is quite important. Ghostblade also gives you the the advantage of being able to split push better with more attackspeed to break towers and movement speed to get away from being caught while splitting.

Ohmwrecker will allow you to be a menance when split pushing or tower diving. The 30% movement speed bonus while near towers will allow you to be super fast with your upgraded speed ultimate, ghostblade and passive. The extra cdr will also allow you to be capped at 40% for maximum mayhem.

Liandry's torment?
You can opt to switch out a defensive item for liandry's torment in late game. The situation may arise where you need the aoe damage, but building more tanky allows gangplank to do more damage in the long run. Think of how much damage rumble ultimate does with only liandry's torment and sorc boots. Gangplank's ultimate (compared to rumble) lasts longer, is global range, covers a larger area, and has more base damage with the increase rate of fire cannon upgrade. Similar to rumble's ultimate it provides an AOE slow which applies liandry's double power making your ultimate an insanely powerful zoning and damage tool. Also the new barrels can proc liandry's which is crucial in using the items efficiently. The AOE damage and slow will proc the double powered burn on every target hit.

Pros:
Super strong late game
Insane AOE damage allowing for great objective control
No dashes but has tons of movement speed modifiers
Does mix damage and is hard to itemize against.

Cons:
Weak early game
No dashy moves for split pushing