The Round Drum is available for the Thompson and the PPSh-41. It is unlocked after the Marksman III challenge for the Thompson and after the Marksman II challenge for the PPSh. It is the PPSh-41's most common attachment; its high rate of fire means the smaller magazines need frequent reloading; and also, the attachment is unlocked at the Marksman II Challenge, meaning it is easier to unlock.
The increase in magazine size that is given depends on the gun. Generally, it increases magazine size by 50%, but with some weapons it can increase magazine size by a greater amount. The attachment also does not increase starting ammo capacity unless paired with Scavenger Pro. A recommended weapon type to get Extended Magazines for is light machine guns, being that they have enormous ammo capacities, and the best penetration in the game. However, it is not recommended with assault rifles or SMGs, because most players tend to reload after each kill, though it can be useful if the player does a lot of spraying. The P90 becomes almost similar to a light machine gun with Extended Magazines due to its already large 50 round magazine being turned into a 75 round magazine, which allows the user to hold down choke-points, but has a much quicker reload than the LMGs. While using Extended Magazines on the P90, a player can sprint for a longer period than normal (patched on PS3/X360, still available on PC). This allows users to equip another first tier perk instead of Marathon like Sleight of Hand or Bling. The P90 is the only weapon which gains this advantage. The AA-12 in particular has a very small magazine to balance it, so Extended Mags (which doubles its capacity to 16 shells) are extremely popular among those who have unlocked it.
Extended Mags returns in Call of Duty: Black Ops, and is available to the player. It can be used on nearly all weapons. This attachment is not to be confused with Dual Mags, which is another attachment in Call of Duty: Black Ops that decreases reload time by taping 2 magazines together.
As an addition, in singleplayer mode, the attachment increases the maximum ammunition count, by having in reserve the same number of extended magazines as the standard ones. This is not the case in multiplayer though, except in the Wii version, where the reserve ammo is increased just like in the campaign.
Unlike in Call of Duty: Modern Warfare 2, the magazine model changes 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." The M60 is also given the name "M60 Big Ammo." Other weapons will be shown using longer magazines. The exception to this are the M14 and the M60, which do not use a different magazine model.
Due to balancing purposes, a weapon cannot accept Extended Mags and Dual Mags at the same time by using Warlord. Rapid Fire cannot be paired with Extended Mags or Dual Mags either.
The Extended Mags returns in Call of Duty: Modern Warfare 3. It increases magazine capacity, and now in multiplayer it increases the starting ammo count as well, in the same way as the singleplayer mode of Black Ops. It almost always increases magazine capacity by 50%, with only 3 exceptions: the AS50, L118A and MSR at 40% increases (although the ammunition in reserve for these 3 sniper rifles is actually increased by 50%).
It is unlocked at Weapon Proficiency Level 10 for Machine Pistols and certain Pistols, Weapon Proficiency Level 15 for Sniper Rifles, Weapon Proficiency Level 24 for SMGs, Weapon Proficiency Level 25 for LMGs and Weapon Proficiency Level 26 for Assault Rifles and Shotguns.
Prior to the January 25th update, there existed a bug for Extended Mags that when equipped on a Shotgun, the weapon would shoot 50% more pellets per shot, effectively increasing damage and consistency in addition to the standard effects of the attachment. When the Extended Mags attachment was combined with the shotgun-exclusive DamageProficiency, it would produce a staggering 210% increase in the damage output of the base version. It has now been patched.
It is most useful for weapons that have low ammo capacities since it increases magazine size as well as starting ammunition. This can give a player the option to choose a different tier 1 perk. As with this attachment, an average player will usually have enough ammo to last their life without Scavenger.
Extended Mags returns in Call of Duty: Black Ops II as Extended Clip; it increases the magazine's capacity like before. However, unlike the previous games, the capacity is increased by lower than 50%. Fortunately, the attachment still gives extra ammo, for both singleplayer and multiplayer modes. Now, the attachment increases magazine capacity by around 33-35%, but has a downside of increased reload times by 10%.
It was planned to be a perk in singleplayer instead of an attachment, as a singleplayer perk icon of it can be found in the game files.
Due to the fact that the Extended Mags attachment is unlocked via bullet penetration kills in Call of Duty: Modern Warfare 2 for primary weapons, it is therefore the least common attachment seen on primary weapons, just under the Shotgun attachment. The class of weapons Extended Mags is most commonly found on is an Assault Rifle.
All LMGs, except the AUG HBAR, with Extended Mags in Call of Duty: Modern Warfare 2 have so many rounds in one magazine that on the HUD, the bullet count overlaps the white bar that separates ammo and equipment indicators.
In Call of Duty: Black Ops, when Extended Mags is used on the AK-47 or the AK-74u, they use the normal magazines used by the RPK. This is strange because where the RPK's magazines have a 40 round capacity, the Extended Mags for the AK-47 and AK-74u have a 45-round capacity.
Black Ops, in singleplayer mode, is the only game in the series where Extended Mags increases the total reserve ammo for a weapon equipped with it.
In the Wii version of Black Ops' multiplayer, this is the case in multiplayer as well.
In Call of Duty: Black Ops II, it is named the Extended Clip. This is incorrect, as it should be 'magazine', due to the fact the guns affected by the attachment all use magazines rather than stripper clips. This was most likely done for players to more easily differentiate it from Fast Mag.
The Extended Clip Attachment no longer visually extends the length of the Gun Magazine as it did in Call of Duty: Black Ops.