Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
I've added several others to this build which I think create a pretty good team of lots of damage which can tear apart the other team if the tank can do his job. This build is really built more on ideas than actual perfection or intensiveness with regard to detail. I just have been using these builds generally with lots of flexibility.
Kog'Maw is a boss
In early game harass with your spells. Later on, get tons of attack speed with madred's and his arcane barrage. You will destroy. This guide is pretty light because I mostly just wanted to point out differences between an AD carry kog'maw and other AD carries. If you need an intensive guide for that, there's plenty around.
One major difference between my build and other AD builds is that I get arcane barrage after void ooze. This is because in early and mid game, void ooze does more damage relatively and makes your ult easier to hit, resulting in a pretty good amount of quick damage. Bio arcane barrage really shines the most from later mid game on. Think about it like this, if you have a fully leveled bio arcane barrage and hit a guy three times when he has 1000 base health, then you did probably around 200 damage or so. But if you hit him with a void ooze and an artillery shot, you did 300-400 damage. Also void ooze has a longer range than your shots with bio arcane and unlike other skill shots, cannot be blocked by minions. So it makes sense to me at least.
While I have a lot of mpen on this build, he is still an AD carry. However, attack speed is what will really be built, with a lot of damage coming in the form of Madred's and his arcane barrage. So mpen is good :) In the end it really is a lot better than attack speed because that will get massively increased with all kinds of items and it's better than armor penetration because most of your damage will come from magic based damage.
This is a pretty typical mastery build. Like the runes (other than magic penetration) and summoner spells, a lot is up to personal preference. I personally am a large believer in figuring things out for yourself so feel free to alter any aspect of this build if you try it out.
I like the boots at first with health potions because with a long ranged char like kog, it becomes very simple to harass and farm. That speed boost really helps and the potions will help you stay in the lane and get minions (you should be in the middle). Other alternatives for later game include sword of the divine if they have dodgers, banshee's veil if you're getting really hammered by APs, frozen mallet for that extra toughness and a nice little slow to keep them there for the kill etc.
Like many carries/ killer class, there are all kinds of options for summoner spells based on your playing style. I think that personally ghost gives you the best chasing and running capability with exhaust being the most useful for ganks and just general killings. Flash is useful as well for shorter bursts. Cleanse can be extremely useful if they have a lot of cc. It can really be a life saver. I would highly recommend flash or cleanse as possible alternatives. Clarity and heal are terrible late game and that's when kog'maw shines so they just seem like bad choices. Teleport is really unnecessary as Kog'maw isn't really a ganker and you should be able to get in your lane quickly enough if you push the enemy back before recalling so it's usefulness is questionable. Rally and fortify are just dumb. Revive is for losers, you will not be a loser with this. Clairvoyance can be good if you're good with it. I am not. Plus kogmaw's ult is pretty good for poking around. Smite is for junglers. no. Ignite is decent for killing runners but your ult should do fine.
I think this is funny
The title references a honey badger, from a video on college humor. I think it's pretty funny.
Also please comment with votes and actually try out the build before voting.