This property may only be added to a Medium or Heavy Melee or Ranged weapon.
A Penetrating weapon is designed to punch through armour—via narrow points, hardened tips, or high-velocity bolts. When you attack a target with an Armour Rating, reduce that Armour Rating by two, to a minimum of zero for the purposes of resolving your attack roll.
This reduction applies only to determining your degree of success; it does not alter the target’s armour for any other purpose.