The Assault Rifle is a weapon in XCOM: Enemy Unknown. The Assault Rifle is the "bread and butter" weapon in XCOM's armory at the start of the game; it is the only usable primary by Rookies and Supports, and a viable option for Assaults. It is unique in that it has its alphanumeric designation, X-9, mentioned in the Laser Rifle's description.
As the game's starting weapon, this gun works surprisingly well against Sectoids and okay against Thin Men and Floaters. It is not as effective against Seekers, however, and should be ideally quickly replaced.
Overwatch may be recommended if you are searching for enemies, as the low range will be partially negated if an enemy accidentally exposes itself and runs through a trooper's sights.
Once Floaters and other tougher enemies start appearing in missions, it's recommended to prioritize development and manufacture of the Laser Rifle (and later, the Plasma Rifle) as the Assault Rifle's relatively lackluster damage will not be able to keep up, placing XCOM troops at a severe disadvantage. At worst case, the weapon will be able to one to two shot Sectoids, Floaters, and Thin Men on Normal difficulty, making it at least usable until the Muton's appearance in May, by which time the Laser Rifle should have been developed.
XCOM base security uses this weapon, as they are technically rookies in class. As it is quite incapable of dealing with high-level aliens, it is best used to deal with the odd Sectoids that a base security trooper can shoot.
If one is struggling to manage weapon availability by the time EXALT appears, or has Total Loss enabled and does not wish to lose valuable weaponry, it may be of use during the initial anti-EXALT missions, as EXALT operatives have low health and very low accuracy, and as such aren't exactly immediate threats, meaning that low-level B-team operatives deployed with this rifle may still be of some use. This strategy should never be used against EXALT elites, however, and by that time the player should have plenty of reserve Laser Rifles.
The Assault Rifle's outer barrel diameter is disproportionate when compared to its relatively low damage which seems to coincide with a low-caliber 5.56mm NATO round. However, the weapon may just have a thick heavy barrel (similar to many real world match-grade barrels), which would at least improve accuracy by preventing heat-induced barrel warping during sustained fully automatic fire.
XCOM's X-9 assault rifle resembles a hybrid between a M4 Carbine and a Bushmaster ACR, while the EXALT rifle is closer to a G36K, all of which are usually chambered in 5.56mm NATO.