From Guild Wars 2 Wiki
Piercing is a line area of effect property of some projectile attacks that allows them to hit up to five enemies in their path within the maximum range instead of stopping at the first enemy the projectile hits.
 Attacks which pierce
 Traits which grant piercing
- Crack Shot - Rifle and harpoon gun shots pierce. Reduced recharge on rifle and harpoon gun skills.
- Coated Bullets - Pistol shots pierce.
- Piercing Arrows - All arrow attacks pierce targets.
- Unyielding Blast (Life Blast from Death Shroud) - Life Blast pierces and causes vulnerability.
- Projectiles will pierce destructible objects, such as siege weaponry.