Expert Challenges are weapons challenges in Call of Duty 4: Modern Warfare, Call of Duty: Modern Warfare 2 and Call of Duty: World at War, earned by achieving a certain number of kills by headshots with a given weapon. In Call of Duty 4: Modern Warfare and the sequel Call of Duty: Modern Warfare 2 they unlock weapon camouflage for primary weapons, but in Call of Duty: World at War, they merely reward experience points.