Trinity is Garbage
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 see everyone pick trinity first. This doesnt seem to give me any power spike, the pieces all feel weak when compared to even a BF sword, and by the time I have completed it, it fails to give any real power spike.
By maximizing magic penetration, you ensure that your harass does key burst damage against their carries, while you may be less effective against tanks, just try to stay out of harms way.
About Runes- Take pure magic penetration in place of Hybrid if you don't have the runes. Corki does mainly magic damage and mixed damage, so armor pen is 0% or 50% as effective as on other carries.