Extended Mags

The Large Magazine is an attachment available for the MP40, Thompson, Type 100, M1 Carbine, and the PPSh-41 in Call of Duty: World at War multiplayer. The attachment doubles the ammunition capacity of a given weapon.

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, or for the PPSh-41, when the Marksman II challenge is completed.

Multiplayer Tactics
What makes the extended magazines effective for SMGs is that it increases the ammo capacity of the magazine. Having Bandolier on further increases the ammo storage. It is often used in conjunction with the Thompson for its low 20-bullet magazine, and for the PPSh-41 as the small magazine will be eaten up too fast by the gun's extremely high rate of fire. Also, MP40 users can use round drums to spray-and-pray, since the drum gives them half a minute of supressing fire. This attachment allows SMGs to constantly fire on a target; often the user will "spray and pray" in a battle since they have more bullets to spend and less frequent reloading to do.

This attachment can greatly increase its user's ability to remain mobile and engage numerous targets. It is extremely useful for rushers. The larger ammo capacity is also invaluable to players using Double Tap, as the extreme fire rates would quickly diminish the standard magazines; it is very common to see double-tappers using extended magazines.

Call of Duty: Modern Warfare 2
A similar attachment called Extended Magazine was seen in a trailer for Call of Duty: Modern Warfare 2. It is unlocked by getting fourty kills through an object while using Full Metal Jacket. It is compatble with most if not all guns in the game not counting lauchers.