On higher SMOD difficulty levels Combine Soldiers can and will take advantage of this feature against you! The amount of penetration varies between weapons. When the SMOD Weapon/Movement Realism setting is set to 'High', The shots fired from most high-powered weapons will go through thin walls and weaker materials like wood and cardboard. It is recommended to shoot enemies in areas that their shields do not cover when possible. The shields absorb any damage from attacks that impact the shield until it breaks.
On higher SMOD difficulty levels, the Combine will begin to equip riot shields to better defend themselves against attacks.