First, there is no such thing as an "assault rifle."
An assault rifle is a selective fire (selective between semi-automatic, automatic and/or burst fire) rifle that uses an intermediate cartridge and a detachable magazine. Assault rifles are the standard service rifles in most modern armies. Note the difference between the assault rifle and the battle rifle. Assault rifles use smaller cartridges and are used at closer ranges than battle rifles. The larger sized rifle cartridges used in battle rifles make fully automatic fire more difficult. Fully automatic fire refers to an ability for a rifle to fire continuously while the trigger is pressed; "burst-capable" fire refers to an ability of a rifle to fire a small yet fixed multiple number of rounds with but one press of the trigger; in contrast, semi-automatic refers to an ability to fire one round per press of a trigger. The presence of selective fire modes on assault rifles permits more efficient use of rounds to be fired for specific needs, versus having a single mode of operation, such as fully automatic, thereby conserving ammunition while maximizing on-target accuracy and effectiveness.
Assault is something you can do with a firearm (or knife, or hands), not a characteristic of the weapon itself.
Well done! You managed to find a ditionary! But on no! You stopped at the first definition.
You know words have multiple, often relared meanings, right?
a. A military attack, such as one launched against a fortified area or place.
b. The concluding stage of an attack in which close combat occurs with the enemy.
An assult rifle is a weapon used commonly in militry assults.