The Hammerburst is a Locust weapon. In the first part of the original Gears of War you use it while escaping a prison to make it back to the front line. It was the assault rifle that fired in quick six shot bursts; it wasn't very powerful compared to the other weapons.
The Hammerburst is tweaked in Gears of War 2. Now it has had slight changes made to it so that it is no longer a backup weapon to the more powerful and popular Lancer. The gun is now semi-automatic, much more powerful and now has a zooming abilities.
Gears of War 3 further advanced the Hammerburst and differentiated it from the Lancer, by adding an additional zoom (on top of the previous one) that has the player looking down the scope of the gun in first person making it the most accurate rifle only bested by the sniper rifles, the Longshot and Oneshot, in Gears Of War 3.