Call Of Duty 5 : World At War
The exact name depends on the weapon, for the Thompson and the PPSh-41, it is called Round Drum, for the MP40 it is Dual Magazines, and for the Type 100 and the M1 Carbine it is called Box Magazine (though strangely if you have not unlocked the Box Magazine for the Type 100, the Marksman challenge will describe it as an "Extended Clip" instead).
Whatever the name, the attachment is unlocked when the Marksman III-or for the M1 Carbine, the Marksman IV-challenge is completed. The PPSh-41 requires the Marksman II challenge.
Call Of Duty : Modern Warfare 2
The Extended Magazines return in Modern Warfare 2. It's unlocked by getting 40 bullet penetration kills while using Full Metal Jacket rounds, or it is obtained by getting the last marksman challenge for a sidearm or shotgun. It is compatible with all guns in the game except for the launchers, Riot Shield, Desert Eagle, .44 Magnum, Ranger and the Model 1887. The amount of ammo in a magazine that it increases is dependent on the gun. Generally, It increases magazine size by 50%, but with some guns it can increase magazine size by a greater amount.
Call Of Duty : Black Ops
Unlike in Modern Warfare 2, the magazine model changes (in first and third person) when extended mags are used, just like in Call of Duty: World at War. A notable example of this are the HK21 and RPK, where they will be given drum magazines, and be called "HK21 Drum Mag" or "RPK Drum Mag". Other weapons will be shown using longer magazines. The exception to this are the M14, and M60, which do not use a different magazine model.