[jff]

An assault weapon is a legal term concocted by lawmakers who wanted to ban scary-looking guns. An assault rifle is a rifle designed to shoot lower-power, lower-caliber ammunition for use in assaults, as compared to the fewer, larger shots from a typical bolt-action rifle (or, as we like to call them, "Fudd" rifles :)

[betterunix]

If I remember my history correctly, assault rifles replaced "battle rifles," which were similarly designed but fired much heavier and more powerful cartridges (and were necessarily larger and heavier). The way I learned it, armies discovered that typical infantry engagements were at distances of hundreds of yards, rather than the mile-long distances guns like the M1 Garand could fire at, and that the weight of the ammunition and recoil of the guns were making soldiers less effective.