A full metal jacket, or FMJ, is by far the most common bullet type. Common design has a copper jacket encasing a large lead core. This bullet type deforms depending on point of impact and obstructions, rather than expanding ("mushrooming"). This and its fair performance against both flesh and armor make it a popular choice with gunslingers. They never suffer penalties, but don't get the damage bonus of JHP ammo, or the armor-punching power of AP ammo.