Could we have a clarrification about wether the armour rating is doubled before or after the pen value of the weapon is applied.

Simple really - BODMAS (Brackets Orders Division Multiplication Addition Subtraction). That is a key principle of mathematics and it means that you always do your subtraction last unless specified otherwise, so in this instance you mutliply the armour rating and then reduce it by the pen of the dum-dum rounds and the weapon firing them.

Idea being that rounds designed to mess up tissue on soft-targets will have trouble breaking through any armour.