Hi Fearn,
No such thing as dumb questions, only dumb answers
Rapier blades are designed to oppose unarmoured opponents wearing either shirt sleeves or civilian street wear (shirt , waistcoat, doublet).
It will run a man through in these for sure, and likely even light - mid weight leather, but any heavier than this (such as buff coat leather), and I would doubt the effectiveness of a rapier blade for piercing.
Against steel armour of any kind, no chance.
Best tool for sticking leather armour or steel armour gaps in close is a sturdy dagger.
Hope this helps
Macdonald