Assault rifle

An Assault Rifle is a selective-fire firearm using a projectile with a muzzle energy in between those of a full-sized rifle and a smaller submachine-gun or a pistol. Assault rifles are categorized in between light machine guns, which are intended more for sustained automatic fire in a support role, and submachine guns, which fire a pistol cartridge rather than a rifle cartridge.

History
Assault Rifles date back to the end of the Second World War. Germany developed the first assault rifle, a weapon called the Sturmgewehr 1944. Although it was not fielded in the same quantities as the Maschinenpistole 1940 or the Karabiner 98Kurtz, the devastating losses it inflicted upon the advancing Soviet armies encouraged the USSR to develop this new class of weapons. The result was the Russian AK-47, which basically set the definition of an assault rifle. By the time of the Halo trilogy, most assault rifles also house an onboard electronics suite, showing ammo capacity, a compass, and other information, without the use of a HUD.

These could refer to the following: It is also possible that the BR55 is an Assault Rifle, as it doesn't fire a full rifle cartridge and, in trailers, seems capable of fully automatic fire hinting at a selective fire feature that was not included due to gameplay issues.
 * MA2B Assault Rifle - The stripped-down version of the MA5B Assault Rifle.
 * MA3 Assault Rifle - The older variant of the MA5B Assault Rifle.
 * MA5 Series
 * MA5B Individual Combat Weapon System - The standard UNSC armament in from the early-2500's to 2552. It was phased out shortly after the Battle of Reach and the Battle of Installation 04. (Halo: Combat Evolved)
 * MA5C Individual Combat Weapon System - A new MA5 model introduced in late-2552 during the late Battle of Earth and the Battle of Installation 00. (Halo 3)
 * MA5K Carbine - A special version of the MA5B Assault Rifle utilized by the SPARTAN-III's and other Special Operation forces.
 * MA37 Individual Combat Weapon System - A MA5-series rifle used and re-designated by the UNSC Army.
 * BR55 Battle Rifle - Replacing the Assault Rifle in Halo 2, despite not appearing as a complimentary weapon to the MA5B in the canon.
 * BR55HB SR Battle Rifle - Replacing the BR55 in Halo 3; it is almost identical to the BR55.