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1.
An assault rifle is a select fire shoulder arm chambered for an intermediate power rifle cartridge capable of being fed from a detachable magazine.

Typically an assault rifle is the standard issue weapon used by military forces. The United States uses the M16A2/3/4 as its standard rifle, though there has been a movement towards the M4A1 carbine (also an assault rifle).
The "MA5B Assault Rifle" from the popular Xbox game "Halo: Combat Evolved" is not an assault rifle because it is not select fire and it is chambered for a full power rifle cartridge (7.62mm x 51 NATO). It would be more correct to say that it is a magazine fed machine gun.

Some examples of real assault rifles include: the M16 series and the AK/AKM series.
by chriscaffee July 24, 2005
 
2.
a specific type of rifle. it is fully automatic, fed by a high-capacity magazine and has a pistol grip. Assault rifles were designed to have the accuracy and power of a rifle, but the automatic fire and portability of a submachine gun. Assault rifles usually use a shortened rifle cartridge, though some do not. such as the Heckler and Koch G3 and FN FAL.
the world's most famous assault rifle is the AK-47. the first assault rifle was the Nazi-made Sturmgewehr 44.
by Awesome McAwesome October 13, 2007
 
3.
A gun that is between a Sub-machine gun and below a light-machine gun. Usually capable of automatic fire, though not designed for support roles.
XM8,M16,AK-47,M1 GARAND, Sturmgewehr 44,
MY assault rifle is the M16.
by K'slAvA March 15, 2008
 
4.
An assault rifle is the weapon of choice for newbs. They tend to spray wildly with it instead of starting with a BR which requires skill and aim.
Wow! stupid newbs and their assault rifles
by Mr. Sin December 15, 2008