The ACR is first used in the mission Cliffhanger, with a Red Dot Sight, Silencer and Heartbeat Sensor attached (this is impossible in multiplayer because there is a maximum of two attachments per gun), although it is also present in the first mission S.S.D.D. as one choices of weapons for running the Pit, located inside one of the weapon crates with an EOTech Holographic Weapon Sight attached. The design of the ACR in Modern Warfare 2 bears heavy resemblance to the original Masada Rifle design by Magpul (one difference is the charging handle, it has the ACR's charging handle not the Masada's) rather than the actual ACR manufactured by Remington.

In multiplayer, the weapon is unlocked at Level 48. It carries 30 rounds in its magazine, deals 30-20 damage per shot, and has a fire rate of 750 RPM. The ACR, although it bounces when firing, has zero recoil as it resets perfectly before the next shot which makes it ideal for medium and long range combat. Its moderately low damage per second makes it somewhat weak in close quarters combat, but it is still very usable at close range. When the Thermal scope is attached, recoil begins to come into effect which can throw the aim off at medium-long range encounters.

The accuracy of the ACR is sometimes better than a sniper rifle. In a confirmed headshot test with the ACR, after firing the ACR until it was dry of bullets, the ACR hit 178 times out of 180 from a mid-long range point of an average rifle battle. In a close quarters headshot test, the ACR fired all it's bullets perfectly at the head-sized target. At medium range, the ACR hit 179 times out of 180.^{[citation needed]}

As a versatile weapon, most Perks and Attachments work well allowing the ACR to fill a variety of roles. Unlike most weapons, the ACR suffers no noticeable increase in recoil when using the ACOG, however as stated above it has some mild recoil with the Thermal Scope, but still it is controllable. Stopping power is recommended as the ACR has one of the lowest damage for assault rifles, although it's accuracy means that it isn't absolutely necessary, and scavenger is also sometimes used because of the ACR's fast ammo consumption with its rate of fire. The ACR consumes ammo fairly quickly and starts with a relatively small ammunition reserve. However, it is so deadly accurate that running out of ammo due to wasting shots with recoil or sway will rarely be a problem.

The default iron sight is open on the top, with a very accurate cross style front sight. The weapon appears to have a decent zoom using these iron sights, making red dot or holographic sights of questionable value.

In terms of damage output per second, the ACR (7th assault rifle unlocked) has the 2nd lowest of all automatic and semiautomatic rifles because it does same damage as the M4A1 carbine but has slightly less damage drop off at range and fires slower than the M4A1's 800 rpm. The ACR trades low damage per bullet for a moderate rate of fire, clear iron sights (though ACOG and thermal scopes are very effective on this compared most of the other assault rifles and SMGs), no sway, no recoil, and an excellent aim down sights and hip fire accuracy.

Trivia

• On the side of the weapon it says "Maneemagusswa Mamdaero, Cal 5.56 Euro"
• On Infinity Ward's website, the ACR was voted the favorite assault rifle in Modern Warfare 2.
• The Cliffhanger version of the ACR can also be found in Just Like Old Times. It has 3 attachments on it (Heartbeat Sensor, Red Dot Sight and Silencer), which is impossible in-game since the maximum number attachments you can have on a gun is 2. The camouflages are different between these guns; the Cliffhanger version has Arctic Camouflage and the Just Like Old Times version is just black.
• Its fire sound is very similar to that of the SCAR-H, even though they are of different caliber.
• In Just Like Old Times you can have two ACRs with holographic sights at one time, the only difference being their camo patterns.
• In reality the ACR is very flexible as it can be customized inside-out. This means that you can change the gun's internal compartments such as the barrel. In fact, the ACR can be customized to fire 7.62x39mm, 6.5 Grendel, and 6.8 Remington SPC.
• In multiplayer, this gun is much more accurate than it is in single player and Special Ops. For example, while looking down ACOG scope in SP this gun has idle sway comparable with AK-47's with ACOG, but in MP has no sway. Later comparing SP ACR and MP ACR you will see, that Single Player version has minor vertical recoil while full automatic fire. In Multiplayer, the only way for this gun to have ANY recoil is while using Thermal Scope.
• The polymer design of the G36 influenced the ACR.
• Much of the ACR's controls such as the charging handle, fire controls, safety and mag release are ambidextrous, unlike the M4/M4A1 which has controls only meant for right-handed people.
• The ACR appears to be the signature weapon of Shadow Company. While they wield a variety of weapons, the ACR is very common, and vast quantities of them in every imaginable configuration are found at Site Hotel Bravo.
• The ACR appears larger than any assault rifle when seen from 3rd Person.