Piercing

Piercing and concussion are weapon statistics, contained in the weapon component's .txt file.

Both values do define how damaging the weapon is (the higher the value, the more destructive the weapon), but not in the same way.

Piercing

Basically, high piercing weapons deal more damage to the chassis. High piercing weapons are especially fit for Gut-rippers, especially for popups (See Razor tips). They are not very fitted to damage other weapons however, and a head-on collision with a concussion weapon will often given the edge to the concussion weapon, unless the piercing weapon is on flails (razors are also often used on Flail shell spinners

Concussion

High concussion weapons deal more damage to components. As a result, they are a good choice when your weapons are designed for hitting head-on the opponent's weapons (except on flail shell spnners), and probably a better choice on spinners than high-piercing weapons. Of course, high-speed concussion weapons will still damage the chassis, but less than a high-piercing weapon will do.