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, also you will reload slightly slower.

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 increase magazine size by a greater amount.

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 unlock 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.
 * 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. Also, the glass between the blinds that face the middle of the map can easily be seen and shot through when standing on the cubicles adjacent to the blinds.
 * The counters in Terminal can be shot through easily.
 * Favela is filled with thin walls that are easy to penetrate with the FMJ attachment.
 * The lack of health in Hardcore modes makes getting penetration kills with FMJ easy.
 * Because sniper rifles are capable of one shot kills, they can get penetration kills through glass, which other guns cannot because they will break the glass before killing the target.
 * Many maps like Quarry and Sub Base have metal catwalks that can be easily shot through because they can be seen through and do not have any noticeable damage reduction when being shot through.
 * Stopping Power is advised to assist unlocking Extended Magazines as it reduces the amount of shots necessary to kill the target, making it harder for them to escape.
 * In Sub Base there is a room with a generator surrounded by fencing. Camping there and shooting through the fencing makes it possible to get 10-20 penetration kills in one game.

Tactics

 * Some players prefer to use Bling with Extended Magazines as a substitute for Sleight of Hand, as it makes reloads less frequent and decreases the chance of finishing a magazine in the middle of a firefight.
 * Extended Magazines do not increase starting ammo on many weapons, but those that do are increased by only a negligible amount.
 * Extended Magazines are highly recommended with automatic machine pistols.
 * Extended Magazines are most useful during medium range firefights, as missed shots are more common and larger magazines will let the player put more bullets down range before needing to reload, hopefully giving the player the advantage of not having to reload under fire.
 * Extended Magazines double the effective duration of the barrage of the Conor147 Barrett Technique

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 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 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. On weapons with already large magazines (such as the G18), 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, except on shotguns (except the AA-12 which uses a magazine), where each shell must be loaded manually.
 * 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.
 * Assault Rifles are relatively easy to unlock Extended Magazines for because of their high penetration and generally lower recoil.
 * 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 400 kills to unlock Extended Magazines for, making Extended Magazines a rare attachment for shotguns.
 * Submachine guns are relatively difficult to unlock Extended Magazines for because of their low penetration and generally higher recoil.
 * Machine Pistols, pistols, and shotguns have Extended Magazines unlocked by achieving regular kills.