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.
As the game's starting weapon, this gun works surprisingly well against Sectoids and passably against Thin Men and Floaters. It is not as effective against Seekers, however, and ideally it should be quickly replaced with higher technology weapons as more durable enemies begin to appear.
On Normal difficulty, at worst case, the weapon will be able to one to two shot Sectoids, Floaters, and Thin Men, making it at least usable until the Muton's appearance in May, by which time the Laser Rifle should have been developed. On the higher campaign difficulty settings, the Laser Rifle should ideally be ready for production by the time Floaters begin to become common, as the relatively lackluster firepower of the conventional Assault Rifle will put XCOM troops at a severe disadvantage.
Overwatch is recommended if you are searching for enemies in open areas, as the hit chance penalty due to its relatively low range will be partially alleviated by enemies that expose themselves outside of Cover and move through a soldier's field of view when initial contact is made.
XCOM base security uses this weapon, as they are technically from the Rookie class. As it is quite incapable of effectively dealing with high-level aliens, in the event of an incursion, it is best to have the security troops focus on dealing with the odd Sectoids that appear; this will also, in turn, free up the more heavily armed XCOM field operatives to focus their firepower on the tougher aliens.
In an Enemy Within campaign, 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, conventional weapons like the Assault Rifle can still be of use during the initial anti-EXALT missions, as the basic EXALT operatives are similarly armed and have relatively low health and low accuracy. This means that low-level B-team operatives armed with this rifle, and using proper tactics, can still compete. However, this strategy should never be used against the much more skilled and better armed EXALT Elites; by that time, the player should have plenty of reserve Laser Rifles to equip even their second-line XCOM field teams.
The Assault Rifle, unique among the weapons and equipment fielded by XCOM, has its alphanumeric designation ("X-9") mentioned in its description.
The Assault Rifle's outer barrel diameter is disproportionate when compared to its relatively low damage which seems to coincide with the 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 in heavy combat situations by preventing heat-induced barrel warping during sustained fully automatic fire.
XCOM's X-9 assault rifle design visually resembles a hybrid of an M4 Carbine and a Bushmaster ACR, while the EXALT rifle is aesthetically closer to a G36K, all of which are usually chambered in 5.56mm NATO.