ACOG Scope

The ACOG Scope (Advanced Combat Optical Gunsight) is an optic available in Call of Duty 4: Modern Warfare and Call of Duty: Modern Warfare 2.

Call of Duty 4: Modern Warfare
The ACOG scope can be used on any Assault Rifle (excluding the MP44), Light Machine Gun, or Submachine Gun after the player has unlocked all Marksman challenges for it (150 kills). It can also be used on a Sniper Rifle in lieu of its standard telescopic sight after 100 kills. Equipped as the weapon attachment, it replaces the iron sights or sniper scope and provides a 2x zoom. The increased zoom improves visibility over range, but increases idle sway - reducing accuracy. Effective bullet range is not actually affected by this attachment (all guns except shotguns have infinite range). The reticle is a red chevron ( ^ ).

Advantages

 * The ACOG makes tracking moving targets (at long distances) much easier, since the target is magnified and while the gun is moving there is slightly less sway.
 * Increased zoom means better visibility when shooting over long distances.
 * The M40A3's base damage is raised to 75 with an ACOG.
 * It converts sniper rifles into regular rifles, making them powerful semi-automatic or bolt-action substitutes.
 * The G36C has little to no idle sway with ACOG.
 * Some weapons (the M14, M21, Dragunov, R700, M82 and MP5 have reduced recoil.
 * The ACOG makes it easier to use a sniper rifle in mid-range combat.
 * It is faster to zoom in with an ACOG than a regular sniper scope.
 * All of your HUD is visible when zoomed in with an ACOG; this is something that can't be accomplished with a sniper scope.

Drawbacks

 * It has higher sway than iron sights or the Red Dot Sight, making pinpoint shooting difficult.
 * It does not allow sniper rifles to be steadied. Also, when prone, you cannot scope in while moving, which you can do with standard sniper scopes.
 * The opaque surroundings of the scope block the player's peripheral vision.
 * It takes longer to sight through the scope compared to iron sights or the Red Dot Sight.
 * The zoom on a sniper rifle is lowered considerably.
 * The ACOG is not recommended for close quarters combat, as it zooms too close for a visible advantage.
 * Most weapons (AK-47, M4, M16A4, G36C, G3, P90, Skorpion, AK-74u, Mini-Uzi, M249, RPD and M60E4) will suffer slight increase in recoil.
 * Most automatic weapons are very hard to use at long range, even those with low recoil with iron sights and Red Dot Sight.

Call of Duty: Modern Warfare 2
While lumped under the name ACOG, each scope is actually different depending on what weapon it is equipped to. So far there have been three types of scopes described; one with a black crosshair (seen on the ACR in Takedown), one with a metal pin in the center (the SUSAT, used on the L86 LSW), and a holographic ACOG much like the original one. The AUG default scope is another version of the ACOG attachment. In multiplayer, the ACOG does not sway on any guns except for sniper rifles, but it now increases recoil considerably.

Trivia

 * In Call of Duty 4: Modern Warfare the ACOG can be used on nearly every rifle and submachine gun, but in real life it is only designed to be used on the M16 and M4, with special options available for the MP5, Uzi, AUG, M14, and certain other firearms. It is not designed to be accurate on any other weapon, especially not weapons like the AK-47 and Skorpion, which use calibers not in normal service in NATO forces. The TA31RCO, the particular model in Call of Duty 4: Modern Warfare, is designed for use only with the M16 and M4.
 * The ACOG in Modern Warfare 2 is slightly easier to aim with, as it does not have as much sway, and the sights are smaller.