Attachment

Attachments, introduced in Call of Duty 4: Modern Warfare and included in World at War and Call of Duty: Modern Warfare 2, are optional enhancements and accessories for weapons. Some directly affect the characteristics of the gun, such as FMJ and the Foregrip, while many others alter the sight displayed while aiming with the weapon, and still others add new capabilities for the player themselves, such as the Heartbeat Sensor, or enhance or add different weapons as seen with the Tactical Knife and the Masterkey Shotgun. Generally, only one attachment can be used at any time; however, Modern Warfare 2 introduced the perk Bling which allows two attachments to be simultaneously equipped. Certain weapons, especially non-firearm ones such as launchers and Riot Shields, cannot use attachments.

Gunsights
Gunsights are the most common attachment, and replace the standard Iron sight that all guns possess (with the exception of sniper rifles which include a high-power scope. All gunsights improve the view of the player while aiming, either by enhancing the zoom-in effect or allowing for greater visibility of the battlespace, especially underneath the point of aim where the firearm itself usually obscures the view from iron sights. Some scopes, however, can cause certain detriments to balance their benefits: they can lengthen the time it takes to transition from hip-firing to ADS (simulating the adjustment that the eye must make), and some block out all peripheral vision, allowing you to see only through the comparatively small gunsight window while aiming. The types of gunsights are:
 * Red Dot Sight (RDS): Included in the Modern Warfare series. Very common sights due to the ease of unlocking them and their general versatility. It is a small glass sheet surrounded by a very thin metal frame that uses optics technology to display, as the name implies, a red dot over the point of aim. ADS zoom with the RDS is 2x, the same as with iron sights. While the sight's accuracy is very good even at extreme range, the dot does not follow the player's movements instantly - there is a moment after a drastic motion during which the point of aim will not be the point of impact until the red dot settles.
 * MARS Sight: A variant of the RDS, it automatically replaces it when the RDS is equipped for the TAR-21. Its viewing window is circular rather than square, and while it allows for better and easier observation of other attachments such as the Heartbeat Sensor while in ADS, the opaque body of the sight that surrounds the viewing window is much larger and therefore obscures more of the view. It is referred to in singleplayer as the MARS Sight, but is just labelled as a Red Dot Sight in multiplayer.
 * EOTech HWS Holographic Sight: Included in the Modern Warfare series. Similar to the RDS in most respects, and is unlocked by getting kills while using the RDS. The very small red dot in the center of the viewing window is the point of aim/impact, but a larger red circle surrounds the dot, which can tend to obscure details or even the target from long ranges. Both the dot and circle are translucent, however, unlike the RDS's dot. While the RDS is almost perfectly accurate in its point of aim vs. impact, the Holographic Sight is in fact perfect in this regard, simulating its parallax-adjusting optics technology, nor does it have to settle after turns or other adjustments like the RDS does. The body of the sight is slightly larger and therefore obscures slightly worse than the RDS, but the sight overall sits much higher on the weapon than the RDS does, and so the weapon itself obscures significantly less of the shooter's view while aiming. Its aiming zoom, like the RDS and iron sights, is 2x.
 * ACOG: Abbreviation of Advanced Combat Optical Gunsight. Included in the Modern Warfare series, they are the last sight to be unlocked by getting a set amount of kills. While the ACOG is perfectly accurate and enhances zoom during aiming much more than the RDS or Holographic, it causes increased recoil. As a result, the weapons that it is ideal for are those with low base recoil and/or very large initial damage output, and it is tailored overall for long-range engagement (but not as long as with true sniper scopes). It also lengthens the transition time from hip-fire to ADS considerably, and the large body of the sight surrounding the viewport obscures a considerable portion of the screen around the targeted area.
 * SUSAT Sight: Identical to the ACOG except in that the fiber-optic and tritium-illuminated reticule is replaced by a post. It is only found on the L86 LSW, where it is also referred to by the game as an ACOG sight, similar to how the MARS sight is referred to as an RDS.
 * Thermal Scope: Unlocked by getting a set amount of kills while using the ACOG. Behaves as a modified telescopic sight, as it has a powerful zoom, it can be steadied by breath-holding, and it obscures all peripheral vision. It is the only sight that actually modifies what the player is seeing in a way other than simply zooming in on it. Players and other sources of heat glow a distinctive solid white in the sight's picture, while everything else is seen in shades of gray. This makes the thermal sight extremely useful for scanning distant terrain for enemies, as any exposed enemy body part will be readily noticeable. Friendlies also glow white, but their brightness strobes, IE blinks rapidly, to simulate the effect of an IR IFF device that they would carry. When viewing a player using the Cold-Blooded perk through a thermal sight, however, they will appear in shades of gray in the exact same spectrum as the rest of the terrain, which can easily cause them to be overlooked if the shooter is expecting the trademark white glow. Smoke, even the thickest smoke generated by a smoke grenade, is completely transparent through a thermal sight, which makes a smoke grenade an ideal complement to the thermal scope.
 * Aperture Sight: Found in World at War, this sight was obviously created to be an equivalent to the Red Dot Sight, as it does not exist in the real world. It consists of a circular glass sheet encased in metal containing two black lines to form a crosshair. It is unlocked after getting 75 kills with a weapon.
 * Telescopic Sight: These are found in World at War as well, and are the equivalent of the ACOG from the Modern Warfare series, with the main difference being that the aiming chevron is black rather than red.

Modifications
Some attachments modify the use and properties of the weapon besides the sights used to aim it. They are:
 * Suppressor: Included in both WaW and Modern Warfare; commonly referred to as a silencer. By capturing and compressing escaping gases, it reduces overall noise emanating from the barrel during a shot. While a player's location is marked by a bright red dot on the minimaps of all their enemies every time they fire a non-silenced weapon, a silencer allows them to fire at will without being revealed in this way. This is particularly advantageous for players that prefer to find an ideal fighting position and stay there picking off targets of opportunity until they are noticed. The silencer also causes less tracer rounds to be fired, making it far harder for an enemy to deduce the shooter's location by observing the impact of their rounds. Naturally, the drastically reduced sound of their firing also makes it harder for observant enemies to locate them from it. The major drawback of the silencer is that it reduces muzzle velocity and therefore damage, especially at range. Theoretically, however, this effect should be neatly removed by equipping an ACOG or thermal scope, as both increase the range statistic of a weapon to the exact same degree that the silencer reduces it.


 * Foregrip: Included in both the Modern Warfare Series and World at War, and mainly found on Light Machine Guns and Shotguns, as these weapons naturally suffer from the greatest recoil. The foregrip reduces recoil, allowing bursts of fire to stay accurately on target during a much larger firing time.
 * FMJ: Abbreviation for Full Metal Jacket. Rounds constructed with a full metal jacket are much more dense and therefore penetrate solid materials much better, retaining more of their muzzle velocity and therefore flying straighter after exiting them as well as doing more damage to the intended target beyond. They also give a very slight general increase in weapon damage. The FMJ attachment is ideal for maps such as Terminal and Highrise where cover is very common but easy to track enemies behind, and is also flimsy to begin with.
 * Extended Magazines: Unlocked in MW2 after a set amount of kills that are made not only with FMJ equipped, but where the killing round penetrates some solid material to reach the target. As such, they are widely considered the hardest attachment to unlock, especially with weapons that do not lend themselves to blind-firing through cover such as shotguns or sniper rifles. As the name implies, Extended Magazines significantly increase the number of rounds that a weapon can have loaded in a single clip, alleviating much of the need for reloading. The increase varies by weapon from 50% to 100%. However, it is very important to note that the total amount of rounds that a player starts with is not in any way affected by this attachment. The only way to begin with a larger reserve of rounds is to utilize the Scavenger Pro perk. It is also worth noting that, as clip size changes, so does the amount of clips you can carry. (For example, a SCAR-H without the attachment will contain 2 reserve clips each containing 20 bullets, yet with the Extended 30 bullet clips, only one spare is required).
 * Akimbo: Appears in MW2 and causes the player to wield two of the equipped weapon, one in each hand. The aiming button then becomes the trigger for the left-hand gun, making ADS impossible. The hipfire accuracy of the weapons is also significantly reduced. SMGs, Handguns, Machine Pistols and the Model 1887 & Ranger shotguns can be wielded akimbo. This attachment is popular with players that emphasize stealth and/or speed to close with their enemies as fast as possible.
 * Sawed-Off: Appears in WaW on shotguns and, as the name suggets, the top end of the barrel is cut off. This increases the damage of the gun, but at the expense of both accuracy and range. It is not present in Modern Warfare or Modern Warfare 2, however there is the Ranger shotgun, which has already had the barrel sawn off. In British english speaking countries, it is referred to as "Sawn-Off", not "Sawed-Off".

Accessories
Some attachments do not actually alter the weapon they are attached to in any way, but rather alter another capability of the player or add a new capability entirely. They are:
 * Grenade Launcher: Appears in both Modern Warfare games, and can be mounted on all Assault Rifles. They are underslung, meaning that they attach to the rifle along the bottom of its barrel over the handguard, and are designed for the shooter to pull their trigger while holding the weapon's magazine as if it were a pistol grip. It loads and fires only one grenade at a time, and must be drawn as a separate weapon before use. The grenade that it fires is identical to a thrown grenade except for the fact that it explodes instantly upon any impact with a non-breakable entity, unless it impacts something so soon after being fired that its explosion would harm the shooter, in which case it does not explode at all. Even a grenade that does not explode in this way, however, will kill any opponent it hits directly. The flight trajectory of the fired grenade follows an arc that must be accounted for when firing from a distance. While it only comes with two rounds upon spawning, the Scavenger perk allows a player to replenish their grenade ammunition very effectively. It is identical to the Thumper except for the fact that there is no ADS for it. The slang term 'noobtube' has arisen to criticize the grenade launcher and its users as 'noobtubers', to such an extent that the latter is a title that can be earned by using it. This attitude arises from a popular sentiment that relying upon the grenade launcher and/or thumper is a way to compensate for poor aiming reflexes with firearms.
 * Heartbeat Sensor: Appears in both Modern Warfare games and is only available on the larger Primary Weapons; unlocked after 15 kills while using the suppressor on a given weapon. The sensor displays as a small screen at the bottom left of the HUD extending from the weapon. There is a white dot in the middle of the bottom of the sensor that represents the player, and a pulse is shown to periodically emanate from the player every few seconds. The pulse scans the 180 degrees to the front of the player up to a moderate distance ahead of them, and displays friendlies as green dots and hostiles as red. No features other than the locations of other players are displayed on the sensor, and their displayed locations are only refreshed every several seconds during each sensor pulse. The sensor must use some form of IFF system to tell the difference between friendlies and hostiles; currently no such gun attachment exists in real life, though standalone heartbeat sensor devices do exist.
 * Masterkey Shotgun: Appearing for Assault Rifles in MW2; unlocked after getting 20 kills with an attached grenade launcher. It has a better range and base ammo pool than any standalone shotgun. However, it possesses several drawbacks. Each of its shells only fires 6 pellets, compared to other shells' 8, amounting to 75% of their damage output per shot. Its firing rate is also very slow and its breech capacity (the equivalent of magazine size) is only 4 shells, making it very hard to actually kill someone at the weapon's max range.


 * Tactical Knife: Only available on handguns. Causes the player to hold their knife at the ready in their offhand while their handgun is drawn, doubling the speed with which the player stabs using the normal stab command. Useful both for stabbing an enemy just before they would otherwise stab you, as well as for recovering from a stab to attempt another or to move on to another enemy within stabbing distance.