For example, if I have like 81% armor pen and my enemy has 50% armor, if I use an ability on them, will it deal 41% more damage than usual after going through all their armor?? I'm thinking this with the example of gangplank with a last wisperer and barrels.
If you have 80% armor penetration it means you will ignore 80% of an enemy armor when calculating damage.
So for example if an enemy has 300 armor you will ignore 240 points of his armor. The remaining 60 armor is the calculated as his effective armor towards you, which ends up as 37.5% damage reduction.
TL;DR Penetration ignores flat armor, not the percentage you can see next to the armor.
EDIT: In your case it means a target with 50% damage reduction has 100 armor. 81% armor pen makes them have 19 armor towards you. So their damage reduction goes from 50% to 16%.
Hope you get the idea.
