Extended Mags

Extended Magazines (often shortened to "Extended Mags.") is an optional attachment available in Call of Duty: World at War and Modern Warfare 2. This attachment is essentially a high-capacity magazine by another name. Like its real life counter-part, it increases the ammo capacity of the weapon it's attached to around 150%-200%.

Call of Duty: World at War
The Extended Magazine is an attachment available for all submachine Guns and the M1 Carbine 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. The PPSh-41 requires the Marksman II challenge.

Multiplayer Tactics
What makes the extended magazines effective for SMGs is that it decreases the likelihood of being caught reloading. Having Bandolier further increases the ammo storage. It is often used in conjunction with the Thompson for its small 20-round 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 suppressing 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 rounds to spend and less frequent reloading to do, which is a tactic that is frowned upon by more experienced players.

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.

Although using extended magazines is good, you must take into consideration you can not use the Aperture Sight or Silencer, just standard sight.

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, 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 double the magazine size of a gun (such as the RPD and AA-12),

Assault Rifles
M4A1 - 45 (from 30)

FAMAS - 45 (from 30)

SCAR-H - 30 (from 20)

TAR-21 - 45 (from 30)

FAL - 30 (from 20)

M16A4 - 45 (from 30)

ACR - 45 (from 30)

F2000 - 45 (from 30)

AK-47 - 45 (from 30)

Sub Machine Guns
MP5K - 45 (from 30)

UMP45 - 48 (from 32)

Vector - 45 (from 30)

P90 - 75 (from 50)

Mini-Uzi - 48 (from 32)

Light Machine Guns
L86 LSW - 200 (from 100)

RPD - 200 (from 100)

MG4 - 200 (from 100)

AUG HBAR - 63 (from 42)

M240 - 200 (from 100)

Sniper Rifles
Intervention - 10 (from 5)

Barrett .50cal - 15 (from 10)

WA2000 - 12 (from 6)

M21 EBR - 15 (from 10)

Machine Pistols
PP2000 - 30 (from 20)

G18 - 49 (from 33)

M93 Raffica - 30 (from 20)

TMP - 25 (from 15)

Shotguns
SPAS-12 - 16 (from 8)

AA-12 - 16 (from 8)

Striker - 18 (from 12)

M1014 - 6 (from 4)

Handguns
USP .45 - 18 (from 12)

M9 - 25 (from 15)

Obtaining

 * An easy way to attain the Extended Magazines in Modern Warfare 2 is to crouch behind thin cover (i.e. cubicles, drums, etc) in the 3rd person game-type and then spray them with hip-fire, as you can see over the cover.
 * Alternatively, Highrise is a good map for getting penetration kills, as opponents' heads are often visible above cubicles but their bodies can be shot through thin cubicle walls.
 * Alternatively, on Terminal you can hide behind the counter at the check-in, then shoot people who exit the place through the counter that is located mid-way between the two points. This is only advisable in Hardcore mode.
 * Favela is also filled with thin walls that are easy to penetrate with the FMJ attachment.
 * Another option is to play hardcore with a Heartbeat Sensor but this takes some skill to identify enemy positions through walls. In hardcore, fewer shots through the wall are required for the kill, making a lucky shot more likely to kill.
 * Sniper rifles by far are the easiest to get the extended magazine on as they can get a 1 shot kill through glass which counts towards the penetration kills that the gun needs. Also, collateral damage kills do appear to count as "penetration" kills, another reason for the ease of getting extended mags with a sniper.
 * However, some cover, such as walls, are thicker than others, like sheet metal, and some sniper shots don't get a one hit kill, and makes the target get to better cover. This makes it easier for other weapons as automatic fire makes it harder for the opponent to escape.
 * Many maps like Quarry and Sub Base have metal catwalks that you can easily see through. You can camp on top of or below them to easily pick off enemies that walk by.

Tactics

 * The Extended magazines are used best with guns with small magazines to start out with and guns with a high rate of fire, and is recommended to use with Bling unless you like the iron sights of the weapon.
 * Extended magazines may be used to reduce the percentage of downtime from reloading, and therefore a higher capability to survive surprise attacks, however, this role can also be filled by sleight of hand, which additionally benefits the user with faster ADS when the user acquires pro. As extended magazines do not extend starting ammo count, it can lead to novice users firing with more abandon, and thus feeling cheated when they run out of ammunition more quickly, due to the decreased downtime.
 * Extended magazines are highly recommended with automatic pistols, particularly the TMP. They either do not have the accuracy or damage to be effective with such small magazines, and hip firing is advised, thus leading to a necessity of using extended magazines to take at least one enemy down before having to reload. This is also dependent on perks. Users with stopping power will find greater ease in killing with a smaller magazine (obviously), and therefore are not as prone to running out of ammo with an automatic pistol before downing an enemy.
 * The Stopping Power perk is advised to assist unlocking Extended Mags as it reduces the amount of shots necessary to kill the target, making it harder for them to escape.

Trivia

 * Although Extended Magazines increase the capacity of your magazine, it doesn't increase your starting ammo count. For example:
 * With the SCAR-H, you have one 7.62x51mm 20-round magazine in your Assault Rifle and two spares. With Extended Magazines (and without Scavenger Pro), you start off with one 30-rd magazine in your gun and only one spare. (Both loadouts starting with 60 rounds each, more if using Scavenger Pro.)
 * With the RPD and Extended Magazine all of the player's 7.62x39mm ammo is loaded into one very large magazine (and no spares). It is equal to 100 rounds (200 w/ extended mag), plus an additional 300 rounds if using Scavenger Pro; which might be an attractive alternative to Bling or Sleight of Hand.
 * In Modern Warfare 2, the Extended Magazine attachment has no cosmetic effect on the magazine. However, it may be interesting to note that the G18 magazines have already been extended in-game to almost 50% longer than usual. Increasing the ammo count again from 33 to 50, one can only imagine how long and cumbersome the magazines could be.
 * Unlike in World at War, in Modern Warfare 2 having an extended mag does not increase reload time.
 * Extended mags have different difficulty levels on unlocking between weapons. From easiest to hardest:
 * Light Machine Guns are the easiest, having a high rate of fire and damage.
 * Sniper Rifles are somewhat easy, if the player equips a thermal scope, fires through an un-shattered window (cracked is fine), and gets a one-shot kill, it will count as a penetration kill.
 * Machine Pistols only require 150 kills apiece without any need for penetration kills.
 * Handguns need 100 kills apiece in order to unlock extended mags, which may not seem like much but can be tedious for the two that support them.
 * Shotguns require a huge 400 kills in order to unlock extended mags if supported, which can take a huge amount of time.
 * Submachine Guns are more difficult to kill with through a wall due to their low stopping power.
 * Assault Rifles are the hardest to unlock extended mags for, as they have a relatively low rate of fire, and only medium damage. The player may lose their target through the wall.
 * Machine Pistols, pistols, and shotguns have Extended Mags unlocked by achieving regular kills.
 * The SCAR-H and FAL both have Extended Magazines by default in Campaign, even if the gun has two other attachments. For example, in Wolverines!, the player starts out with a SCAR-H with Red Dot Sight and underbarrel shotgun. However, the SCAR-H's magazines will have 30 rounds each, instead of 20. This could have been done to balance the SCAR-H and FAL with the other Campaign guns, as the Extended Magazines attachment is not available for other weapons in the campaign.
 * The FAL is modeled with a 30-round magazine, but only has 20 rounds, therefore Extended Mags could be considered an attachment that is supposed to be on it.
 * In real life the AA-12 can hold up to 32 rounds.