This is my first guide so welcome. Corki is my favorite champion, so I thought he would make a good start. I believe he is very powerful in the right hands. This may not be as in-depth as you like, but I hope it will help you.If you would like to see a guide to AP corki then please leave a comment. I haven't played him mid but if there's enough interest I will try to figure that out and expand this guide. Please don't downrate without leaving a reason why in the comment section. I will continue to add to this guide, but i wanted to release it ASAP.

Pros:

Ult has extreme range and is spamable
Great chase/escape mechanism
True damage and reduces armor
Reveals enemies with q
Very fun to play
Good, Sustainable damage
Gatling gun continues firing through stuns
Cons:

Squishy
Can burn through mana quickly in lane
Ult can be blocked by minions
Must master Gatling Gun to be effective
Hard to maximize effectiveness of his abilities

I focus late game runes to add attack damage and tankiness for fights later on. The armor penetration runes are interchangeable with more scaling attack runes, but this helps against tanks.

You may want to exchange the utility masteries with defense masteries if the build is too squishy for you. However I like the bonus mana for more sustain and spam-ability.

I find life-steal items are useful for sustain and keeping yourself alive in fights, so Bloodthirster and Blade of the Ruined King are useful for the end of the game. Guardian Angel provides a revive and extra armor/magic resist, so that gives you more survive-ability outside of life-steal. Trinity Force gives (obviously) a bunch of extra damage. Then you can choose Furor if your team is doing well, but if you need to defend more or are dying a lot go with homeguard. Your last choice is whether to go with Banshees Veil for playing against high AP or Black Cleaver against high armor. Other options include Mercurial Scimitar if the enemy has high cc, or if you're doing well and want some chase and bonus health, go Frozen Mallet. If you're up against high AD Iceborn Gauntlet is helpful because it grants a bunch of armor and the spellblade effect is also useful.

Your e reduces armor which causes you to do a much greater amount of damage so i max that first. The w is mainly for the utility so the q comes next. You want your ult whenever possible as the extra damage and the quicker stacking increases your damage output.

First of all: Never use w except to escape or chase. That's the #1 rule. Also, make sure to use your e at the beginning of fights to utilize the armor redux AMAP. If you suspect an enemy is in a bush, don't be afraid to fire your missile barrage into it, as this has a low mana cost and a reasonably quick storing time. However, if used too much this can add up so you're out of mana and/or have no charges left. In straight up fights, you can and should use your missile barrage to deal extra damage, but be sure to save about 2-3 charges in case they try to escape. If it is really close then fire away.