Bullets will not be stopped by enemy armor.
"What bulletproof vest?"
This item is only useful when fighting troopers or Vehicles with Armor. This skill can make a difference when fighting high level enemy Raid troops with a sound number of Life Points and armor ratings (namely soldiers). However, most soldiers are better countered with Killshots and vehicles are vulnerable to Launchers. Armor-piercing cartridges can offer a degree of balance between extremes.
With the skill being available from level 7, the player needs to consider all present skills before deciding if this ammunition can be useful. Don't forget that Desert Eagle and Thompson ignore all armor ratings by default, being decent weapons available from level 1.