Kog'Maw is an amazing champ with an amazing skill set. Most people build him as a traditional carry, weak early game, but amazing late game. Other builds put him in the caster role where he dominates early (like most casters), but doesn't have the same carrying ability.

This build gives him both. It requires 2 different play styles and may be a little tricky for some players. But when mastered, he dominates early/mid game as a nuking disabler, and late game he burns through 15% health with every shot, hitting 5 times in 2 seconds, for 75% of the target's max health.



Play like 2 Different Characters
Early game, Kog is a nuking caster with a 4 second, 60% AoE slow that can damage and disable an entire team. He shoots artillery that hits hard from extremely far away, checking bushes, killing people running away, or bombing enemies next to their tower. Immediately after spamming his nukes , he can turn on Bio-Arcane Barrage for strong early game DPS.

During mid game, he starts to change from the nuking disabler, to the careful squishy DPS, tearing people up from behind tanks. He can farm lanes and creep camps instantly with , and becomes one of the best pushers in the game.

Late game, he hangs back, out of range of the enemy, doing 14-15% of a target's max health in magic damage with every shot ( + +400-500 AP), shooting at the fastest speed in the game.



Final Thoughts
I've played Kog'Maw for a long time with the old carry builds. It's a lot of fun. But this build brings so much to early game that I feel gimped if I play the old carry now.

Try it. Practice it. Learn to anticipate your opponents moves until you hit every Void Ooze and Living Artillery skill shot. Learn to farm like crazy and push lanes. Learn to switch from nuker/disabler to DPS going into late game. Do these and you can dominate, not just late game, but the entire time.

Kog'Maw scales well with ability power.

Caustic Spittle and Void Ooze both add 70% AP.
Living Artillery adds 30% AP
Bio-Arcane Barrage adds 0.01% for every 1 AP


The Ardor mastery multiplies ability power by 1.04 and Rabadon's Deathcap's passive multiplies ability power by 1.3 Tests show that they do multiply together.
(Ability Power * 1.04 * 1.3)


Below is the adjusted ability power we get from each item.
"Archmages Savvy" Mastery = 14.6016 (at level 18)
x9 = 37.23408 (at level 18)
= 189.28
= 60.84 (0 stacks), 93.288 (4 stacks), 125.736 (8 stacks)
= 33.8
= 74.36
= 101.4
= 27.04 to 54.08
Baron Buff = 54.08

Add 14.6 from Archmage's Savvy to:
226.5 AP = +
287.3 AP = + + (0)
319.8 AP = + + (4)
352.2 AP = + + (8)
406.3 AP = + + (8) +
386.0 AP = + + (8) +
440.1 AP = + + (8) + +
486.7 AP = + + (8) + +
541.5 AP = + + (8) + + +


Remember, 406 AP means an extra 4.06% of the target's health taken away from Bio-Arcane Barrage.

Kog'Maw needs 231% Bonus Attack Speed to reach the cap of 2.5 attacks per second, at level 18.
Caustic Spittle gives 30%, so 201% is left to fill up. Since this build takes the Ardor Mastery, 192% would reach the hit cap.


The 1st build uses 4(Alacrity mastery) + 32.34 + 32 + 40 + 60 + 22 (required) = 190.34%
The 2nd build uses 4(Alacrity mastery) + 25.5 + 25 + 32 + 40 + 50 = 176.5%
The 3rd build uses 10.2 + 32 + 40 + 50 + 50 = 182.2%



Figure out your own
Late game, we'll definitely take 40 from , so we should fill up about 161% (152% with the Ardor Mastery) from other sources.

Items: 20 , 22 , 25 , 30 , 32 , 40 , 40 , 40 , 50 , 50 , 55 , and 60 .

Runes: 38.1% = x3 + x9 + x9 + x9
10.2% = Greater Quintessence of Attack Speed x 3
15.3% = Greater Mark of Attack Speed x 9
6.84% = Greater Seal of Attack Speed x 9
5.76% = Greater Glyph of Attack Speed x 9

Masteries:
+ 4% "Alacrity" Mastery.
x 4% "Ardor" Mastery = approximately +8 or 9% when you get close to the speed cap.

Always pick items that counter your opponents. Therefore this list should always be changing. However, these are the picks that I often go with and the reasons why.

The 3 builds above are samples of the basic ideas below.
Early to Mid game = Caster using +
Mid to Late Game = Carry using + Attack Speed (while still getting the synergy from )

Early to Mid game = Caster

An amazing opening item. If you harass your opponent as much as they harass you, with this and 21 points in defense, you'll consistently have more health. That gives you the ability to do more last hitting or get first blood. I stopped buying a Health Potion because I rarely needed it with Kog'Maw's range and harassing abilities.
Philosopher's Stone Gives you the much needed MP5 + the laning power of the health regen. You can sell it after having it for 13 minutes to make all of your 800 gold back. You can upgrade it into Eleisa's Miracle if they have heavy CC. I usually hold onto it for a long time, but always think of it as an extra 400 if I need it while at the fountain. Meki Pendant + 2x Health Potions -> Chalice of Harmony is also a good opening buy, especially if you're laning against magic damage. It will let you spam your abilities more than Philosopher's Stone.


Your choice. They both work really well with these builds. See the "Attack Speed" chapter to make sure you get close to the attack speed cap.
Berserker's Greaves are cheaper and boosts Bio-Arcane Barrage + your regular attacks earlier, but Sorcerer's Shoes scale well and makes all of your magic damage (including Bio-Arcane Barrage) hit harder.
Rabadon's Deathcap + runes ( Greater Glyph of Scaling Ability Power x 9) + Masteries ( Archmage's Savvy & Ardor ) gives you 220-230 AP from level 11-14. You now have 2 nukes hitting for 421 each (842 together), a more powerful Living Artillery, and a Bio-Arcane Barrage that already does 4.3% Max health at level 1, shooting at 1.5 times a second.You can now farm extremely easily with Void Ooze + Living Artillery.

Mid Game

This is a good time to buy an Elixir of Brilliance.
This item seems not to fit at first, but strangely, it's the item that makes this build work so damn well. It synergizes with the passives from Rabadon's Deathcap, Caustic Spittle, and the Ardor mastery. It keeps your AP effectively high mid-game, and starts the transition into "attack speed carry" mode.It gives 60.84 AP immediately (45 * 1.3 Rabadon * 1.04 Ardor) and builds up to 125.736 AP (93 * 1.3 * 1.04) very quickly.
Note: Void Ooze gives 2 stacks. It may be a bug.


Almost like a free Madred's
Bio-Arcane Barrage now hits for an extra 3.55%. This is a key point in this build. We focused on AP so our spells hit hard. But we ended up to getting 32% attack speed and hitting for 3.55% Max Health. This is very close to what a Madred's Bloodrazor does. The drawback is that you don't get the 3.55% bonus with your normal attacks. You have to be more skilled with the timing of your .


Most builds would have Madred's Bloodrazor, Malady and 210 extra gold at this point.
+ = 5835 gold, 355 AP, 35 AD, 32 atk spd, 3.55% of target's max health on hit (with active)
+ = 5625 gold, 25 AP, 30 AD, 80 atk spd, 4% of target's max health on hit
Comparison
+ = -210 gold, +330 AP, +5 AD, -58 atk spd, -0.25% of target's max health on hit
+ = +210 gold, -330 AP, -5 AD, +58 atk spd, +0.25% of target's max health on hit

During mid-game, + usually does more damage than + because a very strong + + will hurt more than a slightly stronger .

But this won't last long. If the game isn't over before late game, you're going to need Madred's Bloodrazor & attack speed. Luckily, strong + makes farming lanes and jungles very quick and easy. Because you farm so much better with this build (and hopefully dominate more early/mid game), you can buy them fairly quickly.

Mid to Late Game = Carry

Madred's Bloodrazor + Bio-Arcane Barrage + Attack Speed
Very important for late game. With + you can farm this fairly quickly. With this, you'll be hitting for 13.55% of a target's max health. Buy the Recurve Bow first.
This is a good time to buy an Elixir of Agility.

Pick 1 attack speed item depending on your opponents and their items.
Malady is for magic penetration. Probably the best choice for late game.
Sword of the Divine if facing Jax
Wit's End if getting hit by a lot of magic. You may need an Elixir of Agility to get close to the atk spd cap.

Nashor's Tooth or Phantom Dancer are more expensive than the others, but not bad choices. If high health opponents aren't a problem, you can buy Stinger or Zeal before finishing Madred's Bloodrazor to boost attack speed.
Before choosing or , see if they're stacking MR or Armor.

Long Games

This is a good time to buy elixirs.
Sell the Philosopher's Stone if you still have it.
Frozen Mallet will stop single targets from getting away.
Hextech Gunblade enhances all your damage, and 1 Void Ooze through creeps can heal up most of your health.

Goals
Strengthen your AoE nuke / farming skill / escape mechanism, Void Ooze, early.
Strengthen your single target nuke, Caustic Spittle, next.
Start getting decent auto attack speed from Caustic Spittle and items mid game.
Level up Bio-Arcane Barrage when opponents start getting lots of health, towards late game.

Method
Start with 1 level of Bio-Arcane Barrage to harass with, followed by Void Ooze for the slow, and finally Caustic Spittle for another nuke.

Then level up your nukes.
Each level of Void Ooze & Caustic Spittle hurt for an extra 50. Which one you level up more should be based on your opponents. For example, if laning against Ezreal, level up Caustic Spittle because he'll dodge your Void Ooze. If laning against Karthus, level up Void Ooze so you can slow him and hit him more.


Void Ooze is twice as much mana as Caustic Spittle, but easier to farm with.


Bio-Arcane Barrage is very important late game, but usually upgraded last because 1% max health is not much early game. But if you feel you need range in your lane, feel free to put an extra point or 2 in it. It's certainly not bad.


Usually the order is > > >