I saw in a game of TPA vs CLGblack a puzzling thing on their Corki player: He got Last Whisper, which confused me... why would you have an item that is made less effective by his own e? I understand for high armor targets it may be better, but on corki, with 16 arpen from runes and masteries, and 10 % arpen, attacking with a full spray from Gatling Gun and giving 3 stacks of The Black Cleaver means you reduce their armor by a full 85, then you penetrate 16, so you penetrate 101 armor and penetrate an additional 10%. But what may confuse some Corki players is the "fine print" on his E: "The armor reduction persists as long as the enemy is under continuous fire". This is semi-ambiguous. Does this mean if you turn away from them for a second it disappears? Does it persist for the duration? Whatever the answer, the choice is obvious: Last Whisper is the much better buy, as BOTH methods of reduction are only temporary (the full effect of the armor reduction path only lasts at max 4 seconds and is only single target), and are not immediately beneficial. Verdict: Buy Last Whisper.