The Assault rifle is one of two weapons you start off with by default. The other is the shield gun.
The maximum ammo this weapon can carry is 200 bullets (400 with two rifles) and 8 grenades (16 with two rifles).
- Killing an enemy with bullets: "B was ventilated by A's assault rifle."
- Killing an enemy with grenades: "B tried to juggle A's grenade."
- Killing yourself with a grenade: "A jumped on his own grenade."
Primary and secondary fire
Primary fire is just a stream of bullets. They are quite accurate, but they do very little damage, and tend to run out quickly. Ammo for this weapon also seems to be a lot rarer than other ammo.
Secondary fire is a grenade which can be shot a surprising distance, and can bounce. Hold down the secondary fire button to fire the grenade further (the meter in the bottom-right will display this). The icons above the ammo count show how many grenades you have left (the bullets and grenades are separate).
The Assault Rifle, by itself, is one of the weakest weapons in the entire game. If you have to fight with this weapon, use the grenades because the bullets are pretty much useless. The grenades are quite bouncy, so you can be quite skilled and bounce them around corners to surprise approaching foes.
You can also pick up a second assault rifle and use two at the same time. This is a lot more useful, although still not that good, since you can't fire two grenades at the same time and the ammo runs out twice as fast. If you can't pick up a second rifle then quickly find a better weapon.