"A general-purpose military weapon for field or urban combat. It fires a three-round burst. Good range, low power, high rate of fire."
The assault rifle is a compromise between a slow-firing rifle and a long-ranged machine-gun. It can keep enemies in check at a range most weapons can't fire at, while still maintaining just enough rate of fire to threaten both targets in cover and those closing the distance. These traits make it a very well-balanced, highly recommended weapon at both the early and later stages of the game.
Unfortunately, its relatively low shot power is a very real drawback, so you may want to upgrade your colonists to the heavy SMG or charge rifle when possible, especially if facing against raiders using personal shields, or heavily-armored mechanoids that can soak up enormous punishment. If you're however looking for an accurate weapon, this weapon generally dominates both the charge rifle by a fair margin and a more tangible accuracy boost over the heavy SMG.
The assault rifle can be built at the machining table.
The assault rifle is based off of the well-known American M16 assault rifle in real life.