Basics first. Primary fire is an ultra-low-damage machine gun, while secondary fire launches a grenade, which can be charged up to fire further. Picking up a second assault rifle allows you to dual wield them, making the weapon slightly more credible.
The primary fire is best used against opponents you suspect are already heavily damaged - although it becomes entirely useless once you get the minigun. There isn't much technique to this - its pure accuracy. The secondary fire is suffiently easy to dodge that in a one-on-one situation it isn't of much use either - though you can get lucky in this kind of situation if you charge up the grenade fully and lead your shot well. However, if you spawn near a couple of people ighting each other, tossing a couple of grenades into the fray can score a cheap frag or two. Ultimately, the assault rifle is a weapon of necessity. The primary fire is soon made obselete by the minigun or link gun, while the secondary fire is pathetic compared to he rocket launcher. Use it to embarass your opponents and kill stylishly, or use it when it's all you have.
Though weak and very short ranged, its pretty good against incoming redeemers, also the grenades can give your enemy something to avoid while you retreat, wield dual assault rifles as much and as often possible, they do increase your firepower noticeably.