Alright. I'm finally back, I took a long Hiatus from LoL and am back now. I finished my testing for Corki in the last week and have overall grown as a player. The build will be tweaked slightly and gameplay mechanics are going to be changed up some, otherwise the guide will mostly stay the same.

Side note, If you don't like Walls of Text then this isn't a good guide for you, this is going to be a bit in depth.

Pros:
Epic Escaping, and some nice Juking
Underestimated Damage
Amazing long range poking
Easy farming, and good late game farming
Most useful AD Passive in the game (According to me.)
Good AoE if you build him for it
Epic Mustache
You can ride a seal. A. Seal.

Cons:
Really Squishy
Easily out harassed early on due to his short range
Often Focused if doing well.
Bad Farming = Awful game
Needs a competent laning partner.

Pros + Cons are pretty even, But I'd give Corki about a 7 on the Difficulty scale, because his short range and positioning in fights need some practice to get down.

Reds

• Greater Mark of desolation Very Standard AD carry Marks, can be switched for the AD ones if you so desire but I find these more useful
• Greater Mark of Attack Speed Flat Attack Speed, allowing you to get a little aggressive, or farm faster, at those early levels.

Yellows

• Greater Seal of Armor I find these to be Alright for seals, but they're not excellent
• Greater Seal of Replenishment These seals can be used as a mix with other seals or on their own. I suggest mixing them with some other more useful seals, as they fall off late game and Corki's mana problems will be solved by then. Hopefully. (Which I'm not :P)
• Greater Seal of Attack Speed Same as the Reds, but these would be a good choice for you if you are fine on your Mana
• Greater Seal of Attack Damage/ Greater Seal of Scaling Attack Damage These are like the Alacrity seals, if you think you'll be fine without the Mana Regen, the difference is these will scale well for you into the late game, giving you a better attack damage setup, because you will have alot of Attack Speed already.

Blues

Alright, I'm going to be honest with you, AD carry Blues send me into a tailspin (Pun Intended, Cuz it's Corki... and he's a pilot... Haha.... Ha...) Anyways:

• Greater Glyph of Scaling Magic Resist These are my favorites, because they give Magic Resist, and most enemy Champions have some Magic damage and they don't make me think too hard about blues.
• Greater Glyph of Scaling Mana Regeneration It's a pretty nice Glyph if you have minor Mana problems and don't think you'll need the Magic Resist.
  In all honesty, most of the useful AD Runes are in Marks and Seals, so the Glyphs can be used for anything you want, those 2 are just what I find useful for Corki.

Purples

• Greater Quintessence of Desolation Pretty Standard AD Quintessence, paired with the Desolation Reds, and ANY Armor Pen item, plus your Gatling gun, tanks will Run with their Tail/Poison/Rockness/Sword/Shield/Lance Between their Legs.
• Greater Quintessence of Scaling Attack Damage These give you an extra 13.5 AD, which is nothing to sneeze at.
• Greater Quintessence of Attack Speed Once again, these are nice for the Attack speed,
  
  
  On a final don't get Quints in Might if you have them in Seals/Glyphs, and don't go for more then 1 Alacrity set. (I.E. Getting Alacrity Marks and Quints would be a waste)

I'm not really gonna go TO In depth on the Masteries, because with the new Masteries, you will probably want to experiment with your own setup.

• Brute Force Standard for AD Carries going Offensive
• Butcher I know that this is used mainly for Jungling, but It helps me take down the last hits for the first 3 or 4 levels, when 4 damage can make the difference.
• Alacrity Again, pretty Standard for AD
• Weapon Expertise If you're gonna get Alacrity...
• Deadliness Why not Havoc ? Because I love Crit with Corki, however, if you're gonna build into a more "pew pew shred them" damage style, then definately go with Havoc
• Lethality If you go with the Shred them Sustained Corki, not the Burst Crit path, then take the point you have left from this and throw it into Vampirism, Otherwise, this is great for Crits on Corki.
• Vampirism combined with the Doran's Blade will let you take 5-6% Lifesteal to start the game off.
• Sunder Armor Penetration! 1/2 off!
• Executioner If you put 20 points into the Offense tree and then skip this...
  Now that you have the Offense tree all set, you can do a few things.
• Fill Resistance And Hardiness then put the last 3 in Vigor
• Put 2 points into Resistance And Hardiness then go into Durability and Veteran's Scars
• Put the last 9 points into the Offensive tree, however you want.
• Go into Runic Affinity If you like to take Jungle Buffs alot

Anything you want to do with Masteries will probably work well, (Unless you go all AP as AD Corki, but, Y'know... Common Sense and whatnot.) Play around with it, have some fun.

Since the everchanging meta changed, I'm going to fix this section. I don't feel like adding in the icons right now so feel free to skip this.
I have found on a champion like Corki, Flash + Ignite work out very well, because with Corki you're going to be getting in there with your W and using your E to the fullest, so you can use ignite without a problem to finish them off. If you were to change anything up, say you don't like ignite, then go Heal or Exhaust depending on what your support took. Most of the other spells are bad on Corki, but I've never been one for Surge.

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Hextech Shrapnel Shells: This Passive is great for an AD carry, the True Damage will go straight through Armor, no matter how much of it they have, Bear in mind that if you start the game with say, 80 Damage, you will be doing 8 points of True Damage no matter what.

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Phosphorus Bomb: This skill will be one of two things for you. This will either be your amazing early game poke, or your early game brush checking ability.

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Valkyrie: This skill is basically you kicking it into Overdriving and going wherever you need to go, it can get you over walls, right up next to champions, or out of the way of a CC about to land on your face. Use it well.

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Gatling Gun: This skill will be your Bread and butter in the end of the game, and at level 9 if you max it first. The Sheer terror this will bring when an enemy is 1v1ing you and just realized they lost their armor... At level 9, if you max this first, you will be taking at minimum around 20 armor off of enemies, that's in 2 seconds. GG.

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Missile Barrage: This is your Poke, your heavy damaging skill, your shortest cooldown, and most Unique skill: Every 4th time you launch one of these, it deals 50% extra damage, in a nice large AoE Which softens them up pretty nicely. Other then that, it's a tough skillshot to land, but if they're near a minion wave, there's no penalty for nailing the minions and letting the AoE get them.

Okay, so, Maxing Gattling gun first will allow you to win a fight 1v1 and send the enemy packing, Valkyrie is maxed last because we only use it for escapes/chasing, seeing as how we're AD, we also don't focus on Phosphorus because this build is geared for end game Kills, early game farm. HOWEVER if you want to go for a high poke early game then you would want to go: This way you will have a high Poke with your Phosphorus, seeing that you land it, and your E will still help you out some, just don't get in a sustained 1v1 fight, get in, do your poke, get out.
More testing after several patches I've found my preferred way is the second, just because of the damage it does. You want to use the Q to poke and the E to shred that armor. Imo anyways do whatever suits you.

I will be going over item choices, a core build and some extra items.
You're going to want to start with either Boots + 3 Health Potionif you won't be laning with someone who can heal you, or Doran's Sword If you will be with a healer.
On your 1st B get your Boots and either a Null-Magic Mantle or a Cloth Armor if you're being harassed by AD or AP, otherwise go straight for Berserker's Greaves if you're dominating. At this point, you're gonna rush your Bloodthirster To give you your sustain, now, assess the situation, if you are doing very well, no fear that anyone will best you and you want to get even more aggressive, go right on into your Trinity Force However, if you are not doing so well in your survivability you might want to go into a Guardian Angel if you need armor and MR and want the second chance sort of thing, Frozen Heart If the enemy team is super AD heavy, or Banshee's Veil for an AP heavy team. If you rushed your Trinity Force go into your Defensive item, according to the enemy team's composition. If you went into your defensive item then grab your Trinity Force
Now then, The last 2 items, have many different ways of being used, so I'll list a few things to counter situations.

The enemy team has gone all Tanky high Armor! Help! Easy to counter, you get yourself a Phantom Dancer And a Madred's Bloodrazor Those tanks will not know what hit them.
The enemy team is now starting to Focus me down!
You're gonna want to grab some defensive stuff, again go according to the enemy team, AP heavy, AD heavy, and so on, but a good item for Defense on Corki would be, In my opinion, a Frozen Mallet and a Last Whisper The Last Whisper will put your Armor Pen over the edge, while the Frozen Mallet + Other Defensive item (I highly recommend the Guardian Angel if you're being Focused hard, because in the heat of the moment, you see a corki at 1 HP you're gonan be spam clicking him, at least at lower levels, and that will give your team some time to kill them.) Will keep you kicking.
My team is doing well at protecting me, but I don't know what I should build. At this point, you can experiment, I like the Phantom Dancer + Infinity Edge build myself, but you can also go into some other items, like, Black Cleaver + Last Whisper For Super Armor Pen
Or Black Cleaver + Madred's Bloodrazor Just play around with the last few items, have some fun with it, you gotta have fun with the game. :P
Something I have to Add in before I start the Rennovations on this section:
Quicksilver Sash If you want to swap out one of your 2 end game choices for this bad boy, which is basically Cleanse on a shorter Cooldown and some Magic Resist, so that you can go into something like Exhaust or Ignite for yourself, if you prefer this to Cleanse then, More power to you.

Farming on Corki is Amazingly easy, like, seriously if you can't farm with Corki you should play support. So, with Corki, at level one, you will generally do a bonus of 7-10 bonus damage depending on your runes with your Hextech Shrapnel Shells So if you start with 85 damage, which is where I usually am, you will do Around 87 damage depending on your Armor Pen and which minion you're hitting. The only reason you shouldn't be able to farm is because the other lane has more range then you and you're being pushed hard, (In which case your jungler should be there...) Here's a what you'll want to aim for around the 15 minute mark.

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Good: 100 - 125 (This is my goal in every non-support game, and I reccomend
you strive to reach this goal too, because this is a good 2500 gold or, 8 champion kills)

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Not Bad: 75-99 You're getting there, but you could use some work

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Okay: 50-74 There's room for improvement but it's a start

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Bad: 25-49 You really need to reconsider your choice of role in these games.

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WTF: 0-24 Unacceptable. LEAVE MY PRESENCE

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Alright, I'm going to keep this one Short and Simple:

Early Game

You want to see who you're laning with and grab the associated items, You'll want to work on your Farm for your first 6 or so levels, Then start poking with your Ult, try to stay in lane as much as you can, and don't be stupid, if you're being denied farm, let it happen, you can farm in the late game as long as you don't feed in the early game.

Mid Game

At this point you're gonna want to be staying with your team, wherever you're starting to be, continue to farm if there's nothing happening, and ask your jungler if you can take buffs time to time, they both help alot

Late Game

Team fights will be breaking out left and right, which is where you want to be careful, DO NOT charge straight into a team fight, count out in your head, One Mississippi, Two Mississippi... Then jump into the fight, get close enough to be hitting them with Gattling gun, but be able to Valkyrie out if things get bad. Phosphorus bomb will help the fight, and keep on the Missile Barrages.

REALLY Late game

At this point it's 60:00 into the game and you're at full build (I hope...) What you can do here, is swap your Beserker's Greaves For another Phantom Dancer, or, if you want to, but I'd grab the Mercury Treads if you're getting CCed a lot.

Yes, You. Corki player reading my guide. I talked briefly about this earlier, but let's go a bit more in depth on this.
So you are sitting there, and you and your team are in the showdown with the enemy team. You are (Hopefully) Poking them with your Ult, now, if you think that the fight is about to break out, make sure You have your Big One Loaded up, fire this bad boy right as they're clumping up for Maximum Damage. Then you will want to move in so you're just in range of them and Gatling gun them to bits, keep tossing in a Phosphorus for your AoE and make sure you pick a target to focus down, even with your awesome kit you do some amazing AD.
To Recap, Team fights should go like this:
> >
Then you will use Valkyrie For your Chasing/Escaping, and a note on Valkyrie in team fighting, let's say, you're in a fight next to the destroyed blue team tower, and you're blue team, you realize that everyone on your team is dieing, and they're overpowering you, assuming you are not being focused, you can easily get out of this situation, you're going to want to Valkyrie over into the Wraith Camp, and run for your bottom lane, with Valkyrie getting you right on over the wall, you will be able to out run most of them in time to get to safety.

So I threw in a second guide for the early damage high poke Corki players instead of the Sustain damage low burst Corki people, the general game play of this build will be here, and the rest will be in the following sections.

So the Late game on this build doesn't change from the first build, however, Your early game is changed alot. When you are in your lane, you want to still focus on getting the last hits, but around level 5(Not before or you will be mana starved, unless you're with Soraka) you are going to want to start hitting the enemies with your Phosphorus Bomb and bringing them down on health, don't get in any sustained fights because your damage 1v1 isn't that good yet. In this build you're going to be more of a 1v1 king in the early game, and a team fighting god during the mid game (Levels 11-13 when you're maxing out Gatling Gun)

How to Counter build. It'll help alot in situational times. NEVER. I mean EVER. Use the same build every time unless you're in the same position.

If you have an AD heavy team, (Ex. Nocturne Tryndamere Ashe Annie Nidalee) then you are going to want to build some things to hinder their AD, good examples: Frozen Heart Guardian Angel or Randuin's Omen. Bad Examples: Zhonya's Hourglass Atma's Impaler Sunfire Aegis

If the enemy is AP heavy {Ex. Malzahar Swain Singed Karthus(Karthus Jungle Ftw?) Xin Zhao) Then you want to get things that will handle their high AP burst, and allow you to end them after it's over. Good Example: Banshee's Veil (Included in Main build, good if skipped) Force of Nature(EHHHH. Very EHHHH.) Quicksilver Sash Wit's End (Good in Certain situations, if you get this skip a Phantom Dancer) Bad Examples: There really aren't many. Unless you lack common sense and build an Abyssal Mask or a Lich Bane You should be fine.

Thank you for reading my 1st guide here on Mobafire, I'm sorry there's so much text, and not alot of Pretty stuff, but hey, Maybe if the guide is being used I'll Try to set some of that stuff up. Also a BIG Thank you to JhoiJhoi and her "Making a Guide" Guide, without it I'd be still trying to learn how to code this stuff. Anything you have for suggestions leave me a comment about, any Spelling or Grammatical Errors let me know, and that's about it. Have fun Acing them with the Ace ;)

P.S. Added in a 2nd build, feel free to test it out, and don't take my section on counter building and use it in every situation, adapt, for example, if they are CCing you like crazy and you skipped Cleanse Don't get the Force of Nature Get the Quicksilver Sash. That's it.