Expert (challenges)

Expert Challenges are weapons challenges in Call of Duty 4: Modern Warfare and Call of Duty: World At War. In Call of Duty 4 they unlock weapon camouflage (skins), and in World at War just experience points. The challenges require 25, 75 and 150 headshots with a specific weapon (25, 50 and 100 with the shotguns).

Call of Duty 4
Items/XP unlocked after 25 headshots

Digital Camouflage / 100xp

Items/XP unlocked after 75 headshots (50 for shotguns)

Blue Tiger Camouflage / 500xp

Items/XP unlocked after 150 headshots (100 for shotguns)

Red Tiger Camouflage / 1000 xp

When both Marksman and Expert are completed, the Golden weapons are unlocked for each respective weapon type (Assault, SMG's, etc.) The golden gun in each category are as follows:

Golden Guns
Assault

AK-47

SMG's

Mini-Uzi

LMG's

M60E4

Shotguns

M1014

Sniper Rifles

SVD

Call of Duty: World at War
"It took a lot of memory to add weapon camos and in most cases didn't look good on WWII guns. The memory was used to add/upgrade other gameplay features. We will never update the game to include weapon skins and it is unlikely we will do anything other than give XP for completing those Challenges." — David Vahn Vonderhaar, a developer of World at War, mentioning headshot rewards.

There are no weapons camouflage skins for Call of Duty: World at War. Instead, you only get experience points for completing the Expert Challenges.

Items/XP unlocked after 25 headshots

100 xp

Items/XP unlocked after 75 headshots

500 xp

Items/XP unlocked after 150 headshots

1000 xp