Marksman (Challenges)

Call of Duty 4 Challenges
Marksman's Challenges are weapons challenges that unlock the Red Dot Sight, Silencer, ACOG Scope, and Foregrip (for LMGs and Shotguns) attachments. They require X amount of kills to unlock these attachments. The first challenge for Assault Rifles and SMGs requires 25 kills, which unlocks the Red Dot Sight. The second challenge requires 75 kills, and unlocks the Silencer. The third and final challenge requires 150 kills, and unlocks the ACOG Scope. Shotguns and Sniper Rifles have different attachments and kills required compared to Assault Rifles, LMGs, and SMGs. LMGs unlock Red Dot Sights after 25 kills, Foregrip after 75 kills, and ACOG after 150 kills,but basically it's the same process. Shotguns unlock the Red Dot Sight after 25 kills, and Foregrip after 50 kills. Sniper Rifles just unlock the ACOG Scope after 100 kills. These, along with Expert Challenges are required to unlock Golden weapons.

Call of Duty: World at War Challenges
The Marksman Challenges in Call of Duty 5 have only the slightest differences compared to Call of Duty 4. For example, the Marksman I challenge can be completed with 25 kills with the weapon you have used to complete the challenge, this would unlock things such as an Aperture Sight or a Round Drum. Every weapon in World at War has unlockables for it exept the PTRS-41. Grenades, Pistols and Perks do not have any unlockables for them.