User blog comment:UsoppThePotato/Let Corki shine again!/@comment-8506165-20140216172139/@comment-4022742-20140217144321

"With your E and a black cleaver you get 40 flat + 25% armor penetration"

This statement is false. Neither nor  work in that capacity. Seeing as gatling gun states that affected enemies "have their armor reduced" and that the cleaver states that it "reduces their armor by 5%".

You should note that both these effects fall under "armor shred" as it lowers the actual armor value of the affected foe, anyone hitting it gets benefit from it. Where as flat and percentage penetration is calculated over the remainder of this value.

On a with 400 armor that would result in the following math 400-25%= 300-40= 260 <- This being Rammus his new armor value. From here anyone with Percentage or Flat penetration reduces that 260 armor for them personally even more. A Marksmen with would reduce that 260 armor to 169 armor with any flat penetration subtracted from that number as well.

TL;DR: Don't use penetration when you mean shred, it's confusing.