They ignore 60% of your targets armor. So if you buy a Last Whisper and 5% armor pen in runes or masteries are you basically dealing true damage with them since that is 100% armor pen or am I misunderstanding something here?
Rueian1 votesdiminishing returns you'd probably get about 70-80% from the whole thing.
LW + runes =/= 40% its more like 37-38%
The more sources and higher the pen the less you gain from it.
Trebbi1 votesI dont have a clear answer, but here is what i found out
https://www.reddit.com/r/gangplankmains/comments/3e6pzb/gangplanks_barrels/
Zone of Endless1 votes% stack multiplicatively
That's why Irelia isn't immune to every CC out there
OperationDEEZ11 votesnever forget old elixer of iron
100% tenacity 
uuuuuuugh
CrossXxodia1 votesWith Last Whisper & the 5% penetration mastery you're looking at
100 - 60% = 40
40 - 35% = 26
26 - 5% = ~25
Roughly 75% penetration, so only 15% more than the normal barrel.
