I don't remember his name exactly, but i was spectating a game and he was doing well. This was a 2 ADC set up though.

The thing is all of the builds have a bunch of situational build orders and stuff, so take this with a grain of salt.

( I put in a second build. This is the one that I play and have much higher success rates.)

I have no idea what set up he was running, I am bronze 3 so my ideas are likely not ideal.

After playing corki for a bit a i have changed some.

Penetration rules all. Having the early penetration really helps. I run basically 9x Magic Pen reds an 3x Magic pen Quints (cause hybrid is stupid expensive)

This causes the spells to be very threatening. Attack speed doesnt to much for corki, save for make it easier to last hit.

Even taking the 1600 first part of lindary before finishing essence reaver if you don't have any problems would be reasonable.

I play this build because it focuses around the spells. I find it way easier to hit the spells than it is to use auto attacks. That is why there is no real rush on crit or attack speed. This is why runes and mastery are magic pen. This build makes one's spells Q and R way more deadly, especially when combined with the pen form E. The harassment level is very high which is quality.

Take Magic Pen instead of Hybrid if you dont have the runes.

About gunblade. I don't think spell vamp applies to corki's spells, so his spells only starts to heal him once he gets the full gunblade.

If you would rather harass with spells (corki is good at this with the ultimate range); then I would pick the 2nd build, because otherwise his spells feel very weak without focusing on penetration. If you are a traditional ADC player that is good at getting in the auto-attacks, then the 1st build may work well for you, although I have had limited success with it.